100% bad UX, which I believe to be intentional. Un-disablable garbage that only benefits the spammer. In fact, the chief evolutionary contribution of Facebook to the world may only ever be new forms of spam.
https://developers.facebook.com/docs/app_notifications/
You can turn off notifications from any app very easily.
Notifications are a core part of Facebook. It's like complaining that Twitter doesn't have an option to stop people retweeting you.
If facebook had a (physical)complaints box it would be just out of arms reach above a large rose bush.
The only things that have required me to use FB app forced login is marketing from "free stuffz" vendors.