Earlier quoted context omitted.
Well they decrypt it on your behalf when it's on your device. The point is that couldn't do that without your device/account.
They then go on to say: > While we do back up iMessage and SMS messages for your convenience using iCloud Backup, you can turn it off whenever you want. Sure, I can stop backing my data up - but presuming I don't (like the vast majority of people), does that mean if they got a wiretap order they could just read the data straight from their backup servers? I'm not sure if this was just phrased poorly, or if backing up…
Although in a link in this HN article Apple says backups _are_ encrypted: https://support.apple.com/en-us/HT202303
So, a little confusing...
EDIT
bah, found this:
"Right now, although iCloud backups are encrypted, the keys for the encryption are also stored with Apple. "
https://9to5mac.com/2016/02/25/apple-working-on-stronger-icl...