You'd need to consult a labor lawyer to be sure but that is probably a labor law violation unless the front of house staff was being paid the regular minimum wage and not the credited wage.
IIRC the front of the house participated in the pool (I know because we were irritated with the Mgr getting our tips). The hostesses got min wage, but the managers were salaried (with bonuses).
https://www.dol.gov/whd/regs/compliance/whdfs15.pdf