One of the biggest mysteries surrounding the recent YouTube copyright sweep, is why so many music rights holders have been claiming revenues from makers of video game Let's Plays and even from developers promoting their trailers on YouTube.

Indmusic and TuneCore seem to be two of the most active claimants against video makers, filing multiple claims and, in doing so, diverting revenues away from the game makers and towards themselves and their partners.

The two companies are being cited by multiple video makers including Terry Cavanagh, developer of VVVVVV, who had a copyright infringement flagged against one of his trailers.

Rami Ismail of Vlambeer also called out Indmusic and TuneCore. "Basically, if you make music for games, @indmusic and @TuneCore might be "services" you want to avoid," he posted on Twitter.

Mike Bithell, maker of Thomas Was Alone tweeted that he is "pissed off at @indmusic, they are currently systematically copyright claiming all footage of my game, and monetizing. Daylight robbery."

TuneCore and Indmusic are partners which have set out explicitly to monetize music used on other people's videos, which they (or YouTube's automatic copyright checker Content ID) believe are generated by musicians signed to their services.

Indmusic's YouTube channel promotes its ability to monetize music that "goes viral" on YouTube. TuneCore, which recently partnered with IndMusic, acts as a distribution agent and collects royalties on behalf of indie musicians.

"Anyone who spends time on YouTube knows that people make videos all the time using other artists' music in the background," states the company blurb. "For artists, this means that your music could be heard by millions of people around the world. But often, this also means that you don't get paid for your music."

Indmusic CEO Brandon Martinez, interviewed in a blog post on the TuneCore website, talked about how the companies chase down royalties for their clients. "Properly monetizing your content and, moreover, making money from the use of your music in other people's videos can be a complicated effort," he said.

The company seems to have been active in making claims against video makers who made use of music associated with the "Harlem Shake" craze earlier this year. "It was as simple as properly ingesting the sound recording, composition, and accompanying metadata well ahead of the craze taking off, as well as leveraging a knowledgeable team that could move swiftly resolving issues with claims and disputes," he said. "The biggest issue we ran into was educating users/fans on why we were claiming portions of the music in their videos."

Even so, Indmusic's twitter page has been inundated by complaints. The company is advising irate video makers to contact it via email. Musicians are also being shown how to "whitelist" their music so that video-makers who use their content, with permission, aren't being bothered.

Polygon has contacted Indmusic and TuneCore for comment.