Yes, it is legal for an employer to cut an employee's pay as long as he does not cut it below the higher of state or Federal minimum wage, and in most but not all states, as long as you are notified before you work any hours at the lower rate.
He may also pick and choose who does and does not get a pay cut as long as he does not use a characteristic protected by law as the determining criteria.
The above assumes that no legally binding contract or CBA says otherwise.