If the employee is non-exempt, he has no legal expectation of being paid when he doesn't work. If he is exempt, one of the exceptions to the rule that says exempts must be paid their full salary for every week in which he works, is when he takes a full day or days off for personal reasons. He's not being disciplined; he's being treated the same as any other employee who takes time off work when they have no vacation time available. The law doesn't guarantee paid time when he's subpoena'd - only that he can't be disciplined for it.