Of course they don't have to, but it opens the door to make it easier for existing employees and candidates to ask.

So many people are uncomfortable talking about money, and companies push that it's taboo. This law does a tiny bit to push the information asymmetry back to the employee.

EDIT I've always been a pretty aggressive (not in a bad way) negotiator. So for me, asking why not the max would be question one and probing the answer would be the next set of questions. Maybe this wasn't obvious to those less apt to negotiate?