You don't have to enable it, since Google backs up your photos to their servers by default.
Then they proceed to claim those automatic backups are an "affirmative action" that justifies them scanning the contents of your images as well.
If you don't enable backup, Google Photos randomly reprompts on a regular basis with a "sure would be a shame if something happened to your photos!" modal. It's very easy to accidentally turn it on without noticing when this happens, if one has preemptively clicked where they expect a different UI element to be.
It is not enabled by default if we trust it. In fact, I am not even logged in to Google.
It's not on by default. It asks when you first add your account to Android.
> Google backs up your photos to their servers by default.
You keep saying that, but it remains false. The parents explicitly opted in to sending their photos to Google.
>You don't have to enable it, since Google backs up your photos to their servers by default.
When setting up a new phone (and many times thereafter) they prompt you to enable photos backup. It's not on by default if I remember correctly.