Why a parachute for aerobatics only when carrying a passenger? Per FAR 91.307:


  Unless each occupant of the aircraft is wearing an approved
  parachute, no pilot of a civil aircraft carrying any person (other
  than a crewmember) may execute any intentional maneuver that
  exceeds...


So if I'm flying aerobatics solo, I'm not required to have a parachute. But if I have a passenger, both are required to have one.

What is the rationale for that?

I suppose that in something like a Super Decathlon, where the pilot must exit the plane before the passenger, it would be senseless to have one for the passenger but not the pilot. But is there anything more to this?
Well, as with many issues involving the FAA, the "why" is a bit of a mystery.  Perhaps they thought that the pilot can "lead by example" if a bail-out is needed.  I'd imagine that a lot of passengers would be a little reluctant to jump out of an airplane, but once they see the pilot go I can see them becoming highly motivated!

As far as not needing the parachute if only crew members are on board, the FAA has long held the view that they aren't protecting pilots from doing stupid things to themselves since they are knowledgeable and informed, but their regulations are to protect the "general public", or the passengers in this case, who don't know any better.