FTA >T-Mobile contradicted that clear promise on a separate FAQ page, which said the only real guarantee was that T-Mobile would pay your final month's bill if the company raised the price and you decided to cancel

Same way that credit card companies and websites change their terms every now and then. It's not what you agreed to when you opened the account, but they just do it. Don't like it? Then cancel. They know that canceling credit cards is a pain, you have to go change all your recurring charges and automatic payments, and also closing and opening credit card accounts can ding your credit rating a bit. So most people just bend over and take it.