My understanding is that it was due to DRM issues, XP and above have a very tight layer of DRM management that Zune Pass can tap into to make sure that their content is not being pirated. Mac/linux does not have the kind of DRM management the zune needs to meet the copyright agreement they have with record labels.
I kinda figured it was something like that (it's a pain in the ass to code something on *nix without including something that's GPL'ed, and when you do you have to release source, something the record labels definitely wouldn't like too much)... but the damn thing is so sexy!