I have mandetory beeper duty on weekends. I am not compensated unless I am called in. Is this legal?

Almost certainly it is.

If you are exempt, they don't even have to pay you for coming in as exempt employees do not EVER have to be paid anything over and above their regular salary unless they have a bona fide contract that says otherwise.

If you are non-exempt, unless the limits on your time are considered unduly restrictive, which happens quite rarely in these days of cell phones and pagers, you don't have to be paid except for the time you actually come in.