The backups are enabled by default, on your device, as well as the devices of everyone else you iMessage with.
For iMessage to be end to end secured, you need to turn off iCloud Backup on each and every device you have, and everyone you iMessage with needs to turn off iCloud Backup on each and every device they have. If either you or your conversation partner misses a single device, Apple will get either the message text itself in the backup (if Messages in iCloud is disabled) or the message sync encryption key (if Messages in iCloud is enabled).
In practice, this never happens. Non-e2e iCloud Backup is an iMessage cryptographic backdoor.
A backdoor is a very specific claim that isn't backed up.
Defaults matter.
Apple had a plan to secure it, and (I understand) a partial implementation. It was killed specifically to aid surveillance capabilities. If that isn't a backdoor, I don't know what is.
You're not going to find proof in the EULA.
If you don’t enable backups for iMessages, they stay on your device.
Nowhere does it describe a back door that the FBI has access to.