Most of us are perfectly happy with going to YouTube and streaming that clip of Pookie being exhorted — in language that is not safe for work — to lay waste to his place of work, but some folks may want to watch this video offline, or do some tinkering with the clip. Some YouTube videos are available for download, but most are not, which is why people turn to services that allow you to get your own copy of a streaming video. YouTube, not surprisingly, is not a fan of such services and is dangling the threat of legal action against at least one.

TorrentFreak reports on a legal letter sent to the folks at TubeNinja, a service that lets users download videos from YouTube, Vine, Vimeo, and others.

Is it legal to download these videos? Depends on the video, why you’re downloading, what you’re going to do with the downloaded footage, and who you’re asking.

As we explained in an earlier story on services like PlayOn that allow you to record streams from Netflix, HBO Go, and others, the 1984 “Betamax ruling” by the Supreme Court held that making your own videotaped copy of a TV broadcast for your personal, non-commercial use is not copyright infringement.

Additionally, SCOTUS ruled that VCR manufacturers could not be held liable if their devices are misused to create bootleg copies.

While technology has developed well beyond the VCR, the Betamax ruling has been applied to subsequent devices, which is why PlayOn and others contend that they are not doing anything illegal; recording a movie as its streams from HBO Now is no different from recording it off a live broadcast from HBO, they argue, so long as you don’t go making copies for others, or using the recorded footage in any other commercial format.

Unlike PlayOn, which records a copy of the video as it’s being streamed, meaning it takes two hours to record a two-hour movie, services like TubeNinja download the entire file as quickly as possible. One could argue that this is different from the type of recording that is protected by the Betamax ruling. After all, your VCR could never perfectly replicate a full episode of Cop Rock in only a few minutes.

Aside from having an offline copy to watch at your leisure, there are several reasons why you could legitimately use an otherwise copyright-protected video.

The Fair Use doctrine holds that videos (and other copyrightable works) can be used in criticism, comment, news reporting, education, scholarship, or research, without violating copyright.

So if you’re giving a lecture on the cultural impact of viral videos, you should be able to download a number of the relevant clips and re-edit them together into a new piece for use at your lecture.

These gray areas around the use of downloaded videos, along with the fact that YouTube doesn’t own the copyright to the videos it hosts, explains why Google is dancing around the legal issue in its letter to TubeNinja.

Instead, that letter focuses on violations of YouTube’s terms of service, which prohibit the “downloading of any video from the site” unless it’s accompanied by a “download” button. Additionally, the letter accuses TubeNinja of violating the site’s developers’ terms of service which put restrictions on the use of YouTube’s API (application program interface — how the site interacts with other services).

While the YouTube user who downloads videos with TubeNinja may be — and probably is — in violation of their agreement with YouTube, TubeNinja’s owner says that’s not his problem.

“Our own ToS clearly states that the user is responsible for the legitimacy of the content they use our service for,” he explains to TorrentFreak.

As for claims of violating YouTube’s API, TubeNinja says it doesn’t use the API to obtain the videos, so it has no plans of ceasing the YouTube download functionality.

TorrentFreak notes that, YouTube made similar threats of legal action against another downloader service back in 2012, but that this service remains online and popular four years later.

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