There's nothing illegal about data caps as long as they are made clear. T-Mobile got in trouble here because they marked the plan as 'unlimited'. From the artic

There's nothing illegal about data caps as long as they are made clear. T-Mobile got in trouble here because they marked the plan as 'unlimited'. From the article: "'Consumers should not have to guess whether so-called ‘unlimited’ data plans contain key restrictions, like speed constraints, data caps, and other material limitations,' said FCC Enforcement Bureau Chief Travis LeBlanc in a statement."