If I am already in a contract, and the company alters the price on me during that contract period, how was I capable of making a decision?
The right answer is that the other side broke the contract and voided it, but if it was through a "confusing notice of the fee in a junk mail postcard they’ll throw in the garbage" then I may have missed the window to politely exit the contract.
Then again, if I went to all of the trouble waiting for a cable man to show up and install my new cable, only to have the price altered a few months later, they have not only wasted my time but also set themselves up for a potential run in with the FTC by advertising something as $XXX a year which really turns out to be $YYY a year. Price differences are often what make a consumer choose one service over another so it most certainly matters in that regards.