Hmm, I guess it depends on the employer. In places where I've worked, the per diem is specifically for food, travel and hotels because these are expenditures required for doing the job for the company. If it isn't used, then it's meant to go back to the employer - it's their money after all. They do it that way because it's easier than having to submit expenses and receipts each month but they do expect us to be honest with what it was used for.