I retract everything that I said about "may" versus "can" in class today. I was thinking about something else and confused myself. "May" and "can" can sometimes be used interchangeably (like in this sentence). However, often one needs to make a distinction between permission versus ability.
Some times, if permission can not be obtained, ability is restricted. Therefore, to be sure one has the ability, one may (or is it can) find it necissary to get permission first. Like a prisoner asking if he can walk to the corner store would know that without permission he does not have the ability to even leave his cell.