The ICC Fuel Gas Code, doesn't forbid the use of copper gas lines. And for that reason the jurisdiction I work for does not forbid it currently.
IFGC 403.4.3 Copper and brass. Copper and brass pipe shall not be used if the gas contains more than an average of 0.3 grains of hydrogen sulfide per 100 standard cubic feet of gas (0.7 milligrams per 100 litres). Threaded copper, brass and aluminum pipe shall not be used with gases corrosive to such materials.
However, our gas utility forbids it under all circumstances. If they perform an inspection and find any kind copper line they will not provide service. Doesn't matter if the house was built in the 1920s or it would a major inconvenience for you to replace. It has to go and my City has no authority to demand you get such service.
I think the big reason the gas company doesn't like copper pipe is that it is soft enough you can drive a nail through it. And depending on the installation practice of owner, it might not be easily identified between gas or water lines.
Another thing is that gas comes usually from various sources and therefore the quality of the source might not be uniform. So some areas might have too much hydrogen sulfide in the gas, making it corrosive to the copper lines.
So as @Machavity pointed, out it is probably a liability issue. The installation tech doesn't know the quality of the gas line considering the potentially but perhaps minor risk that the gas quality is poor.