Just because you can get around a policy doesn't make it ineffective. I'm sure that slight barrier reduced the number of people taking screenshots of their bank app 99%. Perfect is the enemy of good.
It just gets user annoyed for no reason, why can't they take a screenshot of their bank account anyway? It just makes no sense. I understand banks like it because they are full of regulatory security which don't make sense in real life, that's probably one more to add to the list.