Netflix has been throttling its video streams on the Verizon Wireless and AT&T mobile networks, reducing the default bitrate to 600kbps in order to help users stay under their data caps.

Netflix's admission yesterday came a week after T-Mobile USA CEO John Legere said that AT&T and Verizon deliver Netflix video at a resolution of only 360p. AT&T and Verizon responded that they don't downgrade the quality of video, and it turns out that Netflix is the one doing the bitrate capping.

Netflix applies this default bitrate worldwide across most mobile carriers and says it has been doing so for more than five years, according to an article in The Wall Street Journal. But Netflix isn't throttling on Sprint and T-Mobile USA, telling the Journal that “historically, those two companies have had more consumer-friendly policies.” Sprint and T-Mobile both offer data plans where customers can exceed their caps without being automatically charged extra fees, but they have to make do with slower speeds the rest of the month unless they purchase more high-speed data.

Netflix also revealed a few details in a blog post.

"We believe restrictive data caps are bad for consumers and the Internet in general, creating a dilemma for those who increasingly rely on their mobile devices for entertainment, work and more," Netflix wrote. "So in an effort to protect our members from overage charges when they exceed mobile data caps, our default bitrate for viewing over mobile networks has been capped globally at 600 kilobits per second. It’s about striking a balance that ensures a good streaming experience while avoiding unplanned fines from mobile providers."

Netflix typically streams at 3Mbps for standard quality and 5Mbps for high definition, at least on non-mobile Internet connections.

AT&T—which has throttled its own unlimited data users for years and is trying to avoid paying a $100 million fine related to the throttling—said it is "outraged" by Netflix's actions.

"We're outraged to learn that Netflix is apparently throttling video for their AT&T customers without their knowledge or consent," AT&T Senior Executive VP Jim Cicconi said in a statement sent to Ars.

We asked Verizon for comment but haven't heard back yet.

Netflix declined to make any further comment when contacted by Ars. The company's 600kbps limit appears to apply regardless of whether a user has a large data plan or even unlimited data. But Netflix will solve that problem beginning in May, when it expects to release a "data saver feature" for mobile apps that will let customers adjust video quality.

"The data saver feature will provide members with more control over their data usage when streaming on mobile networks, allowing them to either stream more video under a smaller data plan or increase their video quality if they have a higher data plan," Netflix said. "We recognize some members may be less sensitive to data caps or subscribe to mobile data plans from carriers that don’t levy penalties for exceeding caps. As we develop new technologies, we want to give all our members the choice to adjust their data consumption settings based on their video preferences and sensitivity to their ISPs data overage charges."

Netflix already lets customers adjust their data usage settings from low to medium or high in their account settings:

But this may only work on home Internet connections rather than mobile ones. Netflix declined to answer our question about whether its mobile apps ignore this setting. But given its public statements and the plan for the new mobile data saver feature, it sounds like AT&T and Verizon customers currently can't choose what Netflix resolution they receive.

On T-Mobile, Netflix is part of the carrier's "Binge On" program that exempts certain video from data caps while limiting streaming quality to about 1.5Mbps or 480p resolution. T-Mobile customers can disable Binge On to get higher resolutions.

Sprint used to limit streaming video to 600kbps, but it removed the limit last year. Sprint had said it imposed the limit to reduce network congestion.

The Federal Communications Commission's network neutrality order bans throttling, but the rule only applies to ISPs—like the mobile carriers—and not to online content providers like Netflix. Netflix has been a strong proponent of net neutrality rules, which are designed to ensure that users aren't prevented from accessing Internet content and that online content providers can deliver their services at whatever quality level they want without being impeded by home Internet providers and mobile carriers.

T-Mobile was criticized by net neutrality advocates for the Binge On throttling, and in response made it easier for customers to opt out and now lets video services remove themselves from the program.