On a legal level, it is how GOG works. They still only sell licenses. You just have the loophole that their installers and the games installed by them will work regardless.
While that may be partly true, (also likely) depending on the county you’re located, they’re not able to revoke the license though.
So in this specific case you having the files makes a world of difference.
Err… You often don’t have the files drm free on Steam. Nor in an installable format (without steam).
Anyhow. Seeing the down votes I’d love for some to elaborate.
Otherwise it just looks like some rampant steam fanboys.