You don't think every carrier does this in some manner? It's called Quality of Service. Data compression, throttling or downgrading quality are just ways to preserve the stability and quality of a network. If I was the other big carriers I would be careful not to stir the pot too much. They don't want to end up in a situation where the pot calls the kettle black. T-Mobile was just up front about it and gave a VERY poor explanation of what they are doing or at least used poor terminology which people are now jumping all over them for. The fact is, ever telecom carrier or internet service provider uses Quality of Service to guarantee service levels.
Example: Are people more willing to complain about their Youtube loading slow or Spotify buffering or calls not going through due to lack of bandwidth.