I would bet its down to copyright holders decisions on that..
Makes me mad. Apple has "exclusives" but half the time u can only purchase them.. not rent them.. which would be basically what u could probably rent them normally anyway. so for me, they are not even excludes because i cannot 'rent' them.
Just because of this and less renting , more buying, i fled renting anything on iTunes now..... Its a shame, but Apple just lost me. Perhaps their "in house" TV show stuff would sweeten the deal, but i doubt it.