Valve has a terms of service that you agree to. If you violate anything within that ToS, they have the right to disable your account. You should know that before you go on a spending spree, and you should definitely know that before you willingly violate the rules.
I think everyone violates them to some degree... I know I recently did for the KotOR fix.
Even when you buy a boxed copy of a game, you don't technically own that game. You purchased a license to play and use that game. You didn't make that software, you don't own any of the data on the disc, or in the box. All you did was purchase a license to use them. If you violate that license, or do not accept it, then you are no longer allowed to use that content. Of course there's nothing they can really do to you if you have a physical copy, but that's besides the point. The point is that whether you purchase it online or via retail, you still only purchase a license to use the data. With it being online, it's easier for them to track you and disable it.
I know that, but them having the ability versus boxed retail being impossible to take back makes all the difference. Plus, court cases on these matters have ruled that no matter what the EULA says, a point of sale purchase of a physical product means ownership. They can put whatever they want in the EULA, it doesn't mean it is legally binding.
Anyway, I was one of the very early adopters of Steam (my account is a 5 digit account, created on the day Steam went live) and I've got almost $1000 in games on it, and I've never had a single problem. I've not had my account hijacked (the only place I ever log in is on Steam itself) and I've never committed any violations to jeopardize my account. That's over 6 years without a single problem.
I've had mine for quite a long time as well, since Half-Life 2 came out, whenever that was. I know my "join date" says 2007 though, because I had support change some things which resulted in that. In any case, I have close to 200 games on it and have spent thousands of dollars there. I never had a problem with the terms and such until my friend had all his games plucked away at the drop of a hat.
I did have my account hacked once, I don't even know how as it was during a period I never used it, when I briefly stopped playing video games all together. When I went back, my passworsd and contact email were changed, and someone was playing one of my games... haha. I got my account back in like 2 days though, by sending them some old credit card info. No big deal, that didn't bother me. It's not the hacks I am worried about... I'm not even worried about VAC since I never play multi.