That is not true of all Steam games. Some games that rely on Steamworks will fail when they lose connection. Others have no real DRM at all. The executables can be run completely independent of Steam.

Spend a year or more in a local without consistent internet access and Steam's solution is not nearly as rosy. Putting Steam in offline mode requires that your first make an internet connection to cache. Even then after a few days, the cache expires and you have to be able to connect again.