Why Facebook allows pedophiles on its platform?

Why facebook allows pedophiles on its platform?
This is a genuine question that i have and i want answers to it! Me and my friends we discovered a group on Facebook called "cuties" full with pictures of innocent girls in very inappropriate postures and many men were commenting there! Very bad comments all directed to those children.
Why Facebook throws in jail people that speak an inconvenient truth to them, but allows this?