TL;DR
Because the wrong of rape is categorically different from the wrong of violating property rights. The latter is about a kind of disturbance of a relation of interest in a thing. That is what the said "above remarks" emphasised, the kind of relation all proprietorial relations share. The authors hold that body and self have a relation between them that is in very important ways different from a proprietary relation, even partly a relation of unalienable, natural "identity". Therefore, they hold that it would be wrong to frame the wrongness of rape in terms of artificial proprietary relations since they are part of a distinct moral/legal framework. Rape does not (only) go against something artificial, dispensible, a relation of arbitrary interest, but the person themselves.
Long answer
Let me summarise analyse the overall argument of the paper since that provides the context of this conclusion. The argument of the third section starts at the following point:
[R]ape is not only a
wrong but a wrong against the person raped, which means that it is
based on her interest. That seems correct. But we are still left
with the question of which interest of hers it is based on.
Thus, the authors start with the assumption that rape was wrong because it goes against a particular interest of the person in question. They then introduce two known theories which provide us with possible candidates, both stating that rape was wrong because of it violating an interest of the person in their own body: interest or value because the person uses their body as a thing and does not want it to be used by others or interest or value because the person identifies themselves with their body, ie. extends the self to include their body. Both of these are proprietory categories, which applied on rape can be described as follows:
The wrongness of rape, according to this reply, comes
of the fact that the victim of rape has a proprietorial interest in, and
derived from that a proprietary right over, her own body. It is
her body, she owns it, nobody else may use it without her say-so.
Rape is none other than the non-consensual ‘borrowing of
sexual organs’. So the right not to be raped is, at base, a
property right or an aspect of a property right.
On use-value
The authors introduce this kind of position as follows:
The importance of property lies basically in the
valuable things we can do with that property that we can’t so
easily do without it. This we will call its use-value.
They go on to explain that if the value of a thing is reduced to use-value, the question arises whether the allocation is correct, ie. whether other persons could not "make more" out of that thing, especially in times of scarcity: use-value is closely linked to utilitarian distributions.
Since the basic value of property is
instrumental, different policies and practices may serve that value
more or less effectively at different times and places, depending
on other prevailing conditions.
This would mean two things for the human body: 1) We would only hold a proprietary right over our own body insofar we can make "good use" of it and 2) consequently, it would be part of economic thinking, ie. potentially to disposal and up to the society to decide upon where it is used best or whether other forms of usage by others would not have higher value. This kind of alienating one's own body seems against common moral intuitions.
On identification-value
Identification is another candidate. It seems more appropriate to take this since it seems closer to the intuition of rape violating something very personal:
They regard property as meaning more, as carrying more significance, than just the
significance imported by its use-value. Their regarding it as
carrying that significance actually endows it with that significance
by changing its social meaning. People are increasingly identifying
with what they have. So on top of its basic use-value, much
more of what people hold now has what we might call
identification-value.
Earlier in the text, the following is made clear:
Aspects of
property rights other than the right to use property are basically
derivative of the use-value of property.
Because of it being derivative of the use-value of property, the very same arguments apply. Identification-value is by no means structurally different, it just extends the very narrow concept of property-by-use. Therefore, it is hard to see how this is any better. Thus, the authors introduce yet another case, the case of pure value of property.
On the pure value of property
Here, they argue that there are cases where there is a burglar stealing something from me for which I do have neither any use, nor do I identify with it, and the burglary remains completely unnoticed. Therefore, the former two cases do not apply to this "pure burglary". In these cases, it is nevertheless a violation of my proprietary rights:
Yet
my property right is violated. Why is this? After all, I have no
interest [...] that comes of either
my use of them or my identification with them. But on top of
that, as we already indicated, there is the co-ordinating value of
property rights in securing use-value and identification-value at
large (ie for people in general). My having this property right
[...], which is violated by the intruding estate
agent, does not come of any use-value or identification-value to
me but of the contribution which my having such a property
right makes to the perpetuation of a system of optimal use-value
coupled, so far as possible, with optimal identification-value.
Thus, the authors claim, the pure burglary in question violates my proprietary rights because it violates the system of property rights I have an interest in.
Now...what does this mean for rape, anyway?
It is now that the authors write your cited passage:
We call this the
pure case for the same reason that we called the pure case of rape
the pure case. It is the case of the wrong and nothing but the
wrong.
The analogues between rape and burglary here are startling,
and may seem to support the idea that rape is wrong because we
own our own bodies. But in fact our remarks tend to undermine
this view. Neither the use-value nor the identification-value
which support our proprietorial relations with things can apply
straightforwardly to our relationship with our own bodies.
The authors seem to say that rape is pure wrong because it goes against some fundamental rights that are foundational for our moral/legal system. Against the analogon they seem to say that the relation to our body is in a very important ways not proprietorial and, accordingly, it would be weird to construe the wrongness of rape in terms of property rights. This is exactly what they do:
[T]he main
point can be seen through the lens of the identification-value.
The identification-value of property holding is the symbolic
value of artificially extending oneself out into the world. But
regarding our bodies there is no question of such an artificial selfextension. There is a longstanding tradition in western
philosophy which diminishes the centrality of the body. The
body becomes something like an arbitrary receptacle in which
the real business of human life – that special inner thing called
‘the self’ – just happens to live. This strikes us as an
unintelligible view. People are, in part, their bodies and their
relationship with their bodies cannot, barring strange
pathological cases (schizophrenia?) or conceptually testing
science-fiction (brains in vats?), be that of artificial self-extension. (bolded mine)
The emphasis here lies in the point that we to not extend our selves to include our bodies in an act of artificial claim. We are our bodies. That is why the reduction of our relations to our bodies into proprietorial terms is misguided: It would mean that our body was some "mere thing" which can be alienated/sold/used/disposed of by others if it so pleases us or is better for the society as a whole. But the authors, with their above remarks and the ones made in the following section, make clear that they think that body and self are related in a manner which is significantly different from a proprietorial relation and that they have to be, at least in part, thought of as identical. That is why the wrongness of rape should not be construed or expressed in terms of proprietorial relation since the wrongness of the violation of property rights is completely based on the relation of interest in a thing.
