Possibly similar to the reason why a mouse did not work on the n900, the necessary parts for it were not implemented (as the developers did not think it would be necessary on a phone, they forgot people like us )
Probably the pulseaudio source they use (if it is one of the open Maemo components) has to be patched to enable a 'normal' behaviour... I mean, using a mouse is normal in the Linux world and it was not there :P