Because the file format used by Apple for HD rentals includes an AC3 stream for 5.1 audio. While the ATV just passes the signal straight to the optical port without modification, a proper solution for a high end Mac should require the ability to decode the stream for discreet channel output. Since QT currently has no ability to handle AC3 in any fashion (discreet or pass-through) your high end Mac would be limited to stereo PLII sound.
HD rentals have only been available for a couple of months with the priority of working correctly with the ATV. The coding and licensing issues with Dolby increases the complexity of a general QT implementation. It my be that the rather then delaying the ATV HD option, the released it as is and will circle back for QT.
But yes, for now it sucks.
I don't think that this is the reason. Its pretty obvious that Apple doesn't want us to see the files we are downloading .The problem is only piracy...
Now Apple has a closed system that makes it difficult for normal users to see what they are actually downloading.