Apparently, the suit is about the FBI classifying Juggalos as a gang:

Horrorcore-rap duo Insane Clown Posse, along with four fans, are suing the Department of Justice and the FBI, demanding that the agencies purge the fan name "Juggalos" from their list of gang members. "Organized crime is by no means part of the Juggalo culture," reads the complaint, filed this morning in federal court in Detroit.

The suit stems from the FBI's National Gang Intelligence Center classification of Juggalos as "a loosely-organized hybrid gang," one with multiple affiliations. Lawyers for ICP and the ACLU claim that the profiling of Juggalos — based on their distinctive clown makeup and Hatchetman tattoos — lacks reasonable suspicion of gang affiliation. As a result, the "unconstitutionally vague" designation has since intimidated many from expressing themselves and denied them protection from unreasonable searches, according to the filing.

"The FBI had the impact they wanted: they scared people away from attending concerts and from affiliating together for the purpose of listening to music," Saura Sahu, an attorney assisting the ACLU of Michigan, tells Rolling Stone. He cited the decline in turnout at the latest Gathering of the Juggalos, ICP's annual five-day festival, where police last August arrested numerous people on drug-related offenses outside the event.

"We don't fit in anywhere," Insane Clown Posse's Violent J tells Rolling Stone. "And when people don't understand you, people fear you. All we're trying to do is be like the Stephen King of music. We like to tell horror stories."