Do new professors typically get paid for course prep done prior to starting their job?
It's also my understanding that your situation is standard.

Basically, you're expected to know how to teach that class already.  Any prep you do before hand amounts to you getting ready to do the job they hired you for, which a normal private sector company doesn't pay you for either.

But really, the explanation doesn't matter so much as the fact that this is widely accepted as normal.