These come from your iDevices encrypted backup in iCloud, not from iMessage servers. I guess if you dont use iCloud and back upp to your local iTunes on your Mac/PC, your messages are a bit more "private".
The backups stored on iCloud are encrypted with your Apple ID password.
Your password can be reset and you don't lose your backups so either Apple has your password in cleartext (so the encrypted backups can be viewed freely) or they have full access.
this seems to only be true if you are using iCloud backup. it is easy to not back up to iCloud if you choose.