The problem with RF based networks is hat a given technology can only support so much bandwidth in a given area. Thus apps like FaceTime can literally saturate the network and make it unreliable.
Data is data. However you highlight an important aspect people either just don't understand or dismiss out of hand. RF networks can only handle so much data and once the users saturate that network service quality plummets. At some point there is nothing more that AT&T can do as towers can only be so close. People can sit here and blame AT&T all they want but honestly it is a good thing AT&T has the balls to say no to apps that could crash the network. It is no surprise at all that they waited for LTE to come along either as it is massively improved technology.
AT&T is a regulated business as such they have an obligation to make sure that the primary reason for a cellular network always works, that is the ability to make phone calls. That means that any person that is a customer can pick up their cell phone and make a call at anytime no matter what the rest of the user base is doing.
It is very easy for these networks to become congested to the point of not being usable due to the physical realities of the technology. This use to happen often with plain old cell technology in the past. For example find yourself in an airport that has just been closed down due to the weather and thousands of people try to phone home all at the same time. That is one I experienced myself but other examples have been noted too.
Frankly I'm happy that AT&T is willing to upset a few idiots to make sure the rest of us can make a call from time to time. They waited for the technology to improve so that FaceTime can actually be supported correctly and made sure that there is a significant cost penalty for excessive users. I'm not sure why anybody would have a problem with that unless selfish is their middle name.
Contrary to popular belief FaceTime doesn't use much data. On WiFi it uses 3MB per minute, it is equivalent to 419kbps. Because these estimates are from WiFi they are most like uncompressed.
Each cell site/radio base station can handle multiple channel based on the technology used. On a LTE capable base station each channel can support published speeds, but not every channel is pegged at full capacity, the pipe going to the base station is generally smaller than combined capacity.
As an example, say 10Mbps each channel * 30 channels = 300 Mbps requirement, data pipe to the cell site could be 200Mbps link.
One a cell site assign a 10Mbps channel to a phone, it can only use up to 10Mbps. There is no question of one device taking up the whole cell site bandwidth. It AT&T published 10Mbps per channel and provided total of 50Mpbs for 30 channels it is their problem.
At that speed within their data plan quota, customer can do whatever they want. So no person with any reasonable telecom knowledge is not going buy into AT&T's BS theorey.