The debate over the potential harm of T-Mobile's Binge On continues, with a new study from Stanford University claiming that the perk violates key net neutrality principles and is "likely illegal." Binge On lets T-Mobile subscribers watch videos from streaming services such as Netflix and Hulu without eating into their data plan. It's proved popular with users, with T-Mobile claiming video views on its network have "more than doubled" since the deal was introduced, but critics say it's creating a tiered internet service, with the Uncarrier given the unfair advantage of deciding who wins and who loses.

"T-Mobile’s Binge On is aptly named — it feels good in the short-term but harms consumers in the long run," says Barbara van Schewick, the net neutrality expert and law professor who authored the Stanford study. "The program limits user choice, distorts competition, stifles innovation, and harms free speech on the Internet. If more ISPs offer similar programs, these harms will only grow worse."

Schewick's paper collects together many of the primary arguments against Binge On, including the allegation that it constrains consumer choice, penalizing customers who want to use unsupported rival services. For example, Binge On supports Vevo TV for watching music videos, but doesn't make YouTube or Vimeo available; it supports Ustream for live-streaming video, but not Meerkat or Periscope. The study also suggests that technical requirements to sign up to Binge On are more complicated than T-Mobile claims, and that the carrier is tilting the playing field to the advantage of larger firms.

"Binge On allows some providers to join easily and creates lasting barriers for others, especially small players, non-commercial providers, and start-ups," writes Schewick. She says that if other ISPs and carriers start offering similar promotions and picking their own zero-rated services, it will "end the era" of "innovation without permission" online.

However, supporters of Binge On do have retorts available. For example, Schewick cites Amazon Prime Video as an example of a prominent video provider that isn't available on Binge On, but T-Mobile actually added the service just yesterday. It's only a single point in what is a wide-ranging and quite subtle debate, but T-Mobile's CEO John Legere would claim that examples like this show how Binge On can be good for everyone — consumers and companies alike.

What happens next, though, is up to the FCC. In 2015, the agency introduced its Open Internet order to protect net neutrality, but it's not clear whether T-Mobile's deal breaks these rules. This legislation includes three "bright-line" rules which companies cannot cross (they can't block content, create fast lanes, or throttle data — although T-Mobile is also accused of that), but the issue of zero-rating is treated more ambiguously, with the FCC saying it will evaluate complaints on a case-by-case basis. Evidence such as Schewick's will be crucial in deciding whether T-Mobile is at fault.