They are discriminating against customers with unlimited data plan which is against fcc regulations.
***NOTE: I will admit I am playing a little Devil's Advocate here. I agree, it is a total bag of rubbish. But I also understand why they are doing this. There isn't one reason, there are multiple. Some are relevant, some are not.***
I think the FCC needs to prove without a doubt that they are legitimately discriminating. I think what AT&T might say to the FCC would make the FCC think before taking action.
***We all know they are doing it to make more money. Just wanted to get that out there.***
Reason 1: Cost per MB (or GB, or TB)
AT&T could argue that while they are a service provider, there is always an exception. The top 5%. The top 5% I am referring to are the users who use abuse the network. They go way past normal usage.
Reason 2: AT&T is trying to buy DirecTV
AT&T had a freeze in the wireless industry. They cancelled a lot of projects. Next year's spending on building and upgrading will be even less than this year's. Without the money to build out, some areas will still be on HSPA. Areas that needs the upgrades where it is already congested are likely to get it. But areas where they planned to deploy AWS or other bands for LTE upgrades might not get them. Overall capacity is going to suffer.
Those are just two ideas that come to mind right away. They are likely to argue you are still getting data. I have been throttled a few times. I can still view email, SMS, MMS and read the forums. I wasn't able to stream video, but I could do the things I had to.
Again, I am not saying this a good thing. I don't agree with it. But it is because they are doing it for the wrong reasons.