Shuffle can hold at most 2GB of music, so conversion may not take forever, reducing the chance that users would call and complain that it is taking two whole days for their computer to transfer songs to their iPods.Why the hell would they offer it for the Shuffle and not the rest of the iPods? The functionality is there, it would probably require them to change 5 lines of code to enable it for all iPods.
STill, a well hidden check button should cut down on complaints while giving the users who know what they are doing the chance to do this. I for one would appreciate it a lot. Maybe I should look into Doubletwist.