This sort of thing is dangerous.
People forget that one of the roles of our justice system is not just to either punish or rehabilitate an offender, but to prevent extrajudicial or vigilante justice. The state prosecutes and incarcerates to stop mob violence (aka. torches and pitchforks) after a crime has been committed. It's to stop people taking the law into their own hands and the streets thus descending into riots, violence and the destruction of property.
In short, the state punishes so you have no reason to.
Crap like this severely undermines that principle. If the state will not prosecute Warboys appropriately nor sentence him appropriately, then it's not such a leap to assume this could stand for many other perpetrators of similar crimes. So then what are victims and their families and surrounding communities to think?
And such a state of affairs is the start of a very short road to people being dragged from their homes by angry mobs, based on false accusations, after a crime has been committed in their area.