In a blog post, T-Mobile CEO John Legere issued a warning to customers "stealing data" from the carrier, calling out a fraction of users that are using workarounds to consume as much as 2TB (yep, terabytes) of cellular data a month. Legere mentioned that these users, who constitute a "fraction of a percent," have been masking mobile tethering and hotspot usage, allowing them to use their cellular plan as a substitute for broadband:

We are going after a small group of users who are stealing data so blatantly and extremely that it is ridiculous.

These violators are going out of their way with all kinds of workarounds to steal more LTE tethered data. They're downloading apps that hide their tether usage, rooting their phones, writing code to mask their activity, etc. They are "hacking" the system to swipe high speed tethered data. These aren't naive amateurs; they are clever hackers who are willfully stealing for their own selfish gain. It's a small group – 1/100 of a percent of our 59 million customers – but some of them are using as much as 2 terabytes (2,000GB!) of data in a month.

T-Mobile already offers a 7GB limit for tethering purposes through its $80 unlimited data plan, following which speeds are throttled. However, there are apps available that give users the ability to conceal tethering activity. A few users have been taking advantage of the workarounds to obtain unlimited tethering bandwidth.

In a detailed FAQ, T-Mobile mentioned that it has developed technology which can detect when customers are undertaking workarounds to mask such activity, and that users who fail to heed warnings about breaking the carrier's terms and conditions will be moved to a tiered plan:

We're warning these customers that they are violating our terms and conditions. If they continue to break the rules at any time, they'll lose access to our Unlimited 4G LTE smartphone data plan and we'll move them to a limited 4G LTE plan. That way, we can enforce our terms and conditions and protect the network experience for all customers.

Legere reiterated that the measures are targeted at users abusing the system, and not an issue centering around throttling data:

These abusers will probably try to distract everyone by waving their arms about throttling data. Make no mistake about it – this is not the same issue. Don't be duped by their sideshow. We are going after every thief, and I am starting with the 3,000 users who know exactly what they are doing. The offenders start hearing from us tomorrow. No more abuse and no risk to the rest of our customers' experience. It's over.

Source: T-Mobile, T-Mobile FAQ