Yes (that's the point of a DRM), but like I said, the DRM is easily broken. Some games can also still use steam features when cracked (like joining lobbies, inviting friends, etc), and it's the same "crack" for every game (not withstanding other DRM the game may have).

With Valve, I'm more concerned of not being able to download the games if they go under, than the DRM on the games I have. Over time, the Steam DRM has also been more permissive than before, as I can now play my "family's" games and they can play mine.