actually, per diem ISN'T totally seperate from pay, in reality. Per diem isn't always paid to people who travel for thier jobs. It is pay, but it is a way to pay people without taxing it, which people like. Per diem is pay, but it is used to offset the cost of living, therefore, it can be paid in a different manner. There are plenty of people who travel and don't get per diem. Think about it. Per diem is not a requirement.
so to say per diem isn't pay seems "not right" to me, although it would be nice if everyone who traveled got diem. if it were law that when people travel they have to be paid travel expenses, then, maybe diem could be considered seperate from pay. if there was no diem, people would just write off thier expenses and make due with the pay received. the pay received would most likely be at a higher rate to compensate for no diem, another piece of evidence that diem truly is pay.