Legally I see that you make a good point but morally I do not support such behavior by any corporation, specially as large and dominant as Twitter and FB.
Baker analogy doesn't quite cut it as there are many other bakers in town, give me 5 other alternatives to Twitter?
Yes, I also agree with you from a moral standpoint that it doesn't seem right, but if you agree to their terms by creating an account, you play by their rules.
Honestly, the point I'm trying to make with the baker analogy is only to understand who has the final say: the company has the final say on what actions they will perform and the customer has the final say on where they spend their time and money.
There are actually plenty of alternatives to Twitter, but obviously these corporations own the largest market share. However, to blame Twitter or Facebook for that is misguided. They are as big as they are because customers have decided to agree to their terms, and if those terms change, Twitter and others have to send updated User Terms, at which point all users have every right to cancel their accounts. Any complaints about a company's policies whilst still being a customer is their own fault. :)
Baker analogy doesn't quite cut it as there are many other bakers in town, give me 5 other alternatives to Twitter?