T-Mobile has been the one carrier who decided to strike back at the other carriers back when AT&T and Verizon were the ones getting rid of unlimited plans by aggressively advertising their Uncarrier movements to turn the mobile industry in the US back into the customers' hands. Yeah, Sprint had unlimited plans too but they weren't necessarily doing so well. They'd lost customers when they decided to make the switch from WiMax to LTE so you could imagine outraged customers who found out their 2 year old phone was not going to have 4G anymore, even after the carrier was promising it was...
Source: http://www.gsmarena.com/tmobile_might_be_pivoting_away_from_unlimited_plans_it_isnt_necessarily_a_bad_thing-news-17352.php
