T-Mobile quantifies which unlimited 4G LTE customers are in risk of getting throttled
But the truth is that T-Mobile hasn't changed its policy at all. It merely has quantified the amount of data use that would put customers in danger of being throttled. It is a lot easier to monitor how much data you've used in a month than it is to figure out if you are in the top 3% of the carrier's data users. As T-Mobile says, this threshold changes every quarter. For this quarter, usage of more than 21GB of data puts you in the top 3% of the operator's data users.
By stating that the policy is being done to manage its network's performance, T-Mobile is not considered to be breaking any net neutrality rules. The FCC allows the throttling to take place as long as it not done as a "business practice." On the other hand, Sprint ended the very same practice of throttling heavy data users using congested networks once the net neutrality rules were passed.
source: T-Mobile, TmoNews via FierceWireless