You typically don't get "ownership rights" when you purchase a game on Steam. You'll typically be purchasing a licence to play the game, which could be taken away at any point. Some Steam games don't include DRM after installation, and you'll truly own those games after downloading them. (you can search for a game here, and find the DRM used) I'd recommend avoiding purchasing games on Steam whenever DRM is included if you want to own the game you'd buy, there are a lot of online stores that sell games without DRM.
That's why the button says "purchase" instead of "buy" it's been a bit since I used Steam, so I had to check to be sure. I think there's a legal loophole there, but I'm not great with English.