Cox Communications, one of the largest telecoms companies in the U.S., is being sued by a pair of music publishers for refusing to disconnect persistent music pirates. Evidence in the case is being provided by Rightscorp, who say that ISPs lose their safe harbor protections if they fail to take action against repeat infringers.

Since February 2013, five U.S. Internet providers have been sending so-called ‘Copyright Alerts’ to subscribers who use BitTorrent to pirate movies, TV-shows and music.

AT&T, Cablevision, Comcast, Time Warner Cable and Verizon are all participants in the scheme while telecoms giant Cox Communications chose to opt out. Instead, the company operates its own “strikes” program but by some rightsholders’ standards, the ISP simply isn’t doing enough.

According to WSJ, two music publishers – BMG Rights Management LLC and Round Hill Music LP – have just sued Cox for failing to disconnect customers who allegedly infringed on the companies’ rights multiple times.

The companies, which control the publishing rights to songs by Katy Perry, The Beatles and David Bowie among others, says that Cox’s inaction means that the company has given up its safe harbor protections under the Digital Millennium Copyright Act.

Of interest is the involvement of Rightscorp in the case. BMG and Round Hill are both clients of the anti-piracy monetization company which sends out cash settlement demands to Internet users on their behalf. Rightscorp has been harvesting data on Cox subscribers and will use it in the case.

The anti-piracy company has stated on numerous occasions that its patent-pending technology enables it to accurately identify repeat infringers. According to the complaint there are “hundreds” of these at Cox, all of which should have been disconnected by the ISP.

Filed Wednesday but not yet available to the public, the complaint says that Cox has actual knowledge of repeat infringement by its subscribers yet has failed to do anything about it.

“..Cox has repeatedly refused to terminate the accounts of repeat infringers. The reason that Cox does not terminate these subscribers and account holders is obvious — it would cause Cox to lose revenue,” the music publishers say.

While BMG and Round Hill insist there is a financial motive for not disconnecting customers, the big question involves the ISP’s obligations under the law and, crucially, the very definition of “repeat infringer” and who gets to determine a subscriber as such.

For example, AT&T previously stated that it would never terminate customer accounts without a court order on the basis that only a court can decide what constitutes a repeat infringement.

By contrast, the current complaint suggests that Rightscorp is qualified to make that determination, even though it’s likely that the company has scant idea of the alleged infringers’ true identities.

While BMG and Round Hill are seeking monetary damages from the ISP, Rightscorp itself has a huge interest in the publishers prevailing as success could boost its revenues elsewhere.

When Rightscorp sends its settlement demands to alleged infringers the company is known to include threats that if subscribers don’t pay, disconnection will be the outcome. While some ISPs do indeed disconnect repeat infringers following Rightscorp claims, a court ruling in hand against Cox would be a powerful tool to force other ISPs down the same route.

Equally, however, a ruling in Cox’s favor could undermine the “muscle” behind Rightscorp’s pay-up-or-get-disconnected threats. But with the company currently teetering on the edge and predicting closure if it can’t get fresh cash injections, one last throw of the dice is probably worth the gamble.