The ACLU says the university violated the First Amendment in May when officials charged Young Americans for Liberty $465 in security fees for bringing gun rights advocate Dick Heller to speak at the campus.
Boise police decided to staff the May event with extra officers to increase safety after school officials found out a community member was encouraging event attendees to openly carry weapons on campus.
The ACLU and the Idaho Freedom Foundation want BSU to refund the student group its money and amend its events policies.
BSU spokesman Greg Hahn says university officials are reviewing the ACLU's letter. Hahn told The Associated Press in earlier reports that it's not unusual for the school to charge for enhanced security to defray costs to students and taxpayers.
Earlier this month, officials with the Idaho Freedom Foundation said they were willing to pursue all options, including a lawsuit, against BSU.
ACLU of Idaho Interim Director Leo Morales said that BSU's current policy allows for broad discretion on whether it will charge groups for costs like security. BSU's event policies allows the university to use content-based restraints on speech, Morales said. However, he added that those types of limitation rarely hold up in court.
"We're willing to help them draft a new policy," Morales said.
BSU faces a separate free speech lawsuit from an anti-abortion organization that claims the university violated the First Amendment by restricting campus protests to a handful of "speech zones."