A company can do this because it has complete descretion over work hours, shifts, etc. It can decide when employees have to work and how long; it can decide who has to work how many hours; it does not have to treat all employees equally, so long as it does not discriminate on the basis of a specifically protected category, such as race, religion, age over 40, disability, or sex; and the company can make any given employee work as many hours as it wants, as long as it pays them overtime for all hours over 40 that they work (assuming, that is, that the employee is not exempt from overtime--and note that hourly workers are never exempt; if the employee is salaried and exempt, like many managers, the company can make them work extra hours without paying any extra money at all).In brief: the law regarding overtime lets a company make any given employee work overtime--it just has to pay the employee overtime, if he or she is not exempt.