It is, unfortunately, perfectly legal. An employer is not required to provide a per diem to employees at all; and a client is not required to provide one to a contractor or vendor. So if the client oil company contracts with the inspection company, and the contract includes a per diem, that is paid to the inspection company; and the inspection company, as your employer, then decides what to do with it, how much to remit to you, etc. It is unfair; but unfair does not always mean illegal.