I'm guessing this feature is intended to prevent two scenarios: A) Grandma accidentally undressing during a FaceTime call (for instance, she forgot to hang up and the call kept recording); and B) Grandma getting on a call with a stranger who wanted to shock her by exposing themselves.

At least, that's what the child protection feature† in current versions of iOS is supposed to do.

† Which this feature clearly evolved from, and the article suggests that it might be a bug that has enabled this for adults as well.