The reason that breaking net neutrality is a problem is that it allows the carriers to pick winners and losers based on special deals and makes it harder for new entrants. T-mobile doesn't require payment from the video/audio providers and doesn't discriminate. You just have to follow their documented guidelines, have legal content, and sign up with them.
They even have one porn site at least (evidence they aren't discriminating based on content). But they don't advertise it.
In other words, they may be breaking the letter of the concept of Net Neutrality but not the spirit.