That sounds a little contradictory. Accepting entails a commitment to a contract of employment as a PhD student. It is never acceptable for university would accept a student and then reject them; why should it be asymmetric from the perspective of the employee?
I believe if you accept an offer, it's binding. When you want to accept another offer, you have to get a written release from the first school. I don't know what the name of the document that states this, but you can easily find it.