From reading the suit, it appears that what they're arguing is that the total fees collected by Warner/Chappell would, if Warner/Chappell had no right to collec

From reading the suit, it appears that what they're arguing is that the total fees collected by Warner/Chappell would, if Warner/Chappell had no right to collect it based on copyright, become damages the class-action plainttiffs would then recover back from Warner/Chappell. There may also be provisions in the California Unfair Competition act which would allow the court to order repayment.