For employers who choose to provide paid time off (PTO) to their employees, such PTO should be considered wages that have been earned, but not yet paid to the employee. As such, once you "earn" the PTO, the employer should not be able to take it away (e.g. "use it or lose it").
Futher, apart from being unfair, these "use it or lose it" policies are nothing more than a garnishment of wages without judicial process. When an employee and employer enter into an employment relationship, an agreement arises: the employee will perform services in the future in exchange for compensation. If vacation days are offered as part of that compensation package and then are taken away, the employer has effectively breached the agreement.