Oh, it makes sense now. But isn't steam
they make their Progam Files folder writeable by everyone
so I don't think Windows can't trust you is a problem here.
No, it allows to override library location in settings. You can install games into different locations if you want.
That's kind of my point! Most games usually get installed into the same root folder by steam which can be easily read or written from (haven't looked into it but I think steam provides some API and as you said steam allows anyone to read and write to). "You" in my original post is the game developer, not the player.
Which can be huge issue on computers with multiple users - one user modifies .exe file that other more privileged user runs. Or just .dll file overriding system functionality, etc.
Are we only talking about the saved files here or the whole game? I'm kind of confused. And again, if the user wants to modify the executable, I can't stop them.