I don't see how this is an issue. Let's say google proudly advertises that chrome is backdoor free. But at the same time they provide a remote desktop solution (aka backdoor) that users can optionally enable. Is this an issue?
The iCloud backups are opt-out, not opt-in, that's the issue. Most people leave settings at their default, and if a company says "We care about your privacy and security", you expect that to be reflected in the default, but here it seems Apple went the other way.
If the messages are encrypted at rest on your phone, it seems reasonable to expect the same of backups on the server.
They are. The issue however is the key escrow.
It’s like locking your front door before you leave to work and let the keys on. Sure it’s locked, but does it matter ?