That is because apple doesn't ask you, it just turns it on. If you don't accept changes to their terms of service (As I haven't) you'd get these notifications fairly regularly.

Granted, I have iCloud turned on (for syncing notes and calendar) but I have it turned off for everything else. I’ve never had it ask me to turn on any of the other iCloud features like iCloud backup or drive / desktop sync. (And I’d guess if it synced things without my knowledge it would show up in my iCloud storage usage?)