Is it against the law not to pay differential pay 4 3rd shift at a University & not be in a union?

i do belong to a union and one way to handle it without messing up the bookkeepers is to work the second shift 7.5 hours and pay them for 8 and the 3rd shift 7 hours and pay them for 8...why in the world would they expect someone to work for the same money as day shift...