I mean, I can copy Baldur's Gate on a PC where there's no Steam at all and play it just fine, because the game itself doesn't have any restrictions
I don't think so, no. You can do that with the gog version. With the steam version it'll try to launch / connect to the local installed steam at startup, and fails if it cannot do so. You'd need to install a steam emulator like goldberg for it to work.
This is the case with most games (there are a few exceptions) on steam, even those that don't enforce "strong" DRM. They want steam running. This is, by itself, a completely unacceptable form of DRM.