I believe that you agreed that they can change the user agreement at any time.
I personally hate these things like Steam. I'll never own anything like a Kindle for the same reason.
As far as I am aware, there are things with Steam that are illegal, however. For instance, it is 100% legal to resell a game, and the line between a normal "license agreement" today and just buying a game is fuzzy at best (after all, you never "owned" a game, ever, you ALWAYS simply had a license, same with books and movies).
Not being a law student or anything like that, I believe, for instance, 2F paragraph 3 is unenforceable by Steam, even if you would have to take them to court to sell or rent a game (you could transfer reproductions without their permission, but it would be illegal unless you transfer it to the person you sold/gave the original to...and I believe it would actually need to be done in that case).
Anyway, if you agreed to something that says they can change the terms at any time, you are bound by those new terms or have to cancel your account. If you did NOT agree to that, then they should be unable to enforce a change, unless you agree to it. That being said, you would likely need to get a lawyer in order to get anywhere.
I'm really not a fan of Steam exactly because it is so unfriendly to the user and will never buy another game that requires it to play (or install, as my CiV box stated, though it is needed for more than that). The only way to really do anything about it would be going to court over any illegal activity, which would likely cause more trouble than it's worth, or simply stop using the service, either for your current games or just avoiding them in the future.
The last option is the least likely to cause you problems personally, but also the least likely to send an immediate message. Still, if you are unhappy and don't have the time or resources to get a lawyer and still want to play a game you paid for, it's the one that comes Rooftrellen suggested.