Ouch, kinda makes it a chore if you'd like to try out the other side for a bit...
EDIT: Of course I can see the reason from a roleplaying perspective, although then I'd expect to see Dark Jedi's get kicked out of the Order and sent packing towards the Empire, eh? But I'm talking about ease-of-play here by letting us mail stuff between our Republic and Empire characters (or have I missed that we're not allowed to have characters from both sides on the same server, perhaps?)...