Is it legal for an employer to require you to stay after your scheduled work hours, if they pay you?

My mother works at a retail outlet and is regularly called upon to stay after he scheduled hours. She is told that she doesn't have a choice in the matter and that they can require her to do so. They have done this in the past, but she had voluntarily stayed after, but then tell her she has to cut her hours the next week. Now they are telling her she has to stay.