Banning is a punishment. It wouldn't be a punishment if you got to keep your qi. If you want to think that way, then when an account is banned, all its stuff should be given to an alt of that player.
Qi transferring used to be around, it lead to a lot of account sales just to take yhe qi and transfer it. Since account sales aren't allowed anymore, how do you think most users are gonna try to get qi?
Edit: I apologize for any typos. Its harder to type on an ipod than a keyboard.