[zchrykng]

Yeah, it is end to end encrypted, but the keys are part of your device's iCloud backup. So unless you turn on end to end encryption for that backup or disable it, Apple can access the keys required to decrypt the iMessage in iCloud messages.

[etchalon]

I believe the reason iMessages aren't protected with iCloud Backup is because they're stored decrypted in the SQLite database iMessage uses, chat.db.