All I had to do to turn my phone into a stealth Google Photos uploader was to turn on the backup sync, then uninstall the app. Whereas one might reasonably believe uninstalling the app from the phone would stop photos from uploading automatically to Google Photos, the device still does it even in the app’s absence. Since making this discovery, I have re-created the issue multiple times in multiple settings on my Galaxy S5.
So, you delete Google Photos and it continues to upload your photos without your consent. Perfect.