Yeah, but the store doesn't actually tell you which is which when you're looking to buy a new game, now does it?!

Further, will that copy of an install folder work if you copy it into a new machine? Maybe, but probably not (it depends on things like how the game handles missing registry keys and/or the graphics card changing whilst there's already a shader cache for the previous graphics cards).

When you're making a purchasing decision, if that factor is very important to you, Steam's possibility that maybe it can be done in an unofficial non-supported way, but you don't get told upfront if it does work, and you're not sure if it will work if you change machines, doesn't count as a real "I get to keep the game no matter what" feature - it's a hack, that sometimes works, usually doesn't.

In GOG that feature is standard.