If this is disabled or archived, this also means that it is still "known" to the system, currently the data simply is not removed from the disk, but other relevant information is purged from the database.
So I agree, that deleting an account should also delete the mailbox data, if there is no proper way to restore that (currently you have to create a new mailbox record with the same mailbox handle and magically the data comes back). For that some archived state could be the solution to have a structured way to restore.