Let me take this from the value proposition angle (I am assuming you own none of these)
- A scoring tube cutter is about $15.
- A simple metal file is $10 or less (or you can buy a de-burr/reamer tool for more than that).
- A Sharkbite 3/8" ball valve itself (just shove it on) is maybe $12 (and it's as close to goof-proof plumbing as you can get)
- A water shutoff key is less than $10 in most cases
So about $50 in hardware. Takes about 10-15 mins to turn your water off, cut the pipe, deburr it, shove the Sharkbite on and turn the water back on. By the time you add all that up, the average homeowner is probably not going to spend that time and money when you can pay some guy who has it on his truck and all the tools to do it right there. Add in the convenience factor, and $100 isn't exactly unreasonable, but you could spend that money to have the tools yourself to replace, say, the other valve under your sink. Or the rest of the valves in your house. And you come out ahead from not paying the installer some $90 to do it. This is a DIY site and... well, we're gonna tell you to buy the tools and do it yourself.
The reason HD likely has a policy like that is these older mutli-turn stop valves do wear out over time. My house is some 40+ years old and I've been steadily doing this over time because some of them just stopped working properly (some needed a wrench to get them to cut off). I recently replaced my kitchen sink and the gaskets inside disintegrated when I tried to turn them off. It's worth your time to buy the tools and replace both valves there (and any others you can reach).