AT&T's contractual obligations have very little to do with whether or not their decision will upset their customers. AT&T is (possibly illegally - the FCC prevented something similar three years ago) decreasing the functionality of a service that they provide.
The FCC issued a rule requiring transparency, not preventing network management. The letter you refer to was a request for information on their policies, not an outright ban.
It is basic human nature that this will upset the folks it affects. It's the same reaction as when your favorite sandwich is eliminated from the menu of the restaurant you eat lunch at 3 times a week; it sucks.
Certainly some people will get upset. ATT may even want them to get so mad they leave or switch plans so they can kill the unlimited tablet plan completely. Sometimes a company doesn't want a particular customer and thus will eliminate what makes them stay a customer or fire them outright. ATT could have simply terminated all those plans at the end of a billing cycle.
What is also coming into play is "loss aversion," people feel much more strongly about losing something than gaining something. In this case, even though they may never be affected by the change they see it as a losing something and thus have a negative reaction.
I suggest you give up trying to moderate the emotions of folks on the internet. While you may be perfectly right about your facts, the facts you are choosing to discuss have little-to-nothing to do with why folks are upset.
Oh I agree, expecting people to be rational, especially on the internet, is a losing proposition.