Salary is compensation. 401K matching is compensation. Vision, dental, and medical insurance is compensation.
Per diem is no more compensation than providing a hotel in the field; that's the company's cost of doing business. It's there to enable the employee to eat and function in the field, but should never be considered part of one's compensation package.
If one manages to save a little per diem, perhaps by eating cheaply or less often, so be it; this isn't the company gracing the employee with extra income; this isn't pay or compensation. The hotel isn't compensation, nor is. the shuttle or the limo to get to the airport, and for ACMI carriers with home basing, the cost of the ticket to and from work isn't compensation either.