It is illegal: if you are non-exempt, that literally means you are not exempt from overtime--that is, when you work more then 40 hours in a work week, you must receive overtime for all all time past 40 hours. Flex or comp time is not a legally valid alternative. Your employer, which controls your hours, is free to not let you work more than 40 hours in a week if it wants--but that's the only way to control overtime. If you do work more 40 hours, you must get overtime. If you don't, you could contact your state department of labor to file a complaint.