Steam does not behave that way. Don't make stuff up.
Excuse me, but it does, and I've seen it with my own two eyes. Block the Steam client from verifying your game, you can't play, period.
You only need to activate the game once with Steam and it's no hassle to do so on more than one computer. You can play Steam games offline.
Sorry, but again, I've seen it firsthand. It does not behave this way and quite frankly I think you steam fanatics know this to be fact and are willfully deceiving people about it.
Are you confusing it with Ubisoft's horrible system? Or how about Securom which used to install itself as malware without letting the user know?
Nope. Not at all. I'm well aware of what Ubisoft DRM does. I'm also equally aware of what SecuROM can do. I'm also aware that only in the worst of worst cases does SecuROM do as you say. Most companies who use it don't use the full package. Like, oh, Bethesda with Fallout 3.
Oh, and I'm also fully aware that Ubisoft style DRM is not the only thing left if Steam goes poof - and you'd be naive to think that impossible.