Apple doesn't allow this because if it's illegally downloaded music, they don't want you to share. Especially with their program (since they might be legally liable for any enfringement). And if it's music bought from iTunes, the file will only work on a system when the song's owner is logged in, so there's no point in sharing that way.
As for legal music other than iTunes, since they can't tell if an mp3 is legal or not, they can't take the chance and assume it's illegal and don't allow it.
What really ticks me off is that I can't drag stuff from my iPod into my iTunes library, even though I can drag from my library into my iPod.