Why are you mad at the parents?

There is no way to tell a child sex predator, unless they have been caught before. Any one of your friends/relatives could easily take advantage of you being away from your child for a bit.
This guy does appear to be a previous sex offender, so there was no indication that parents could have known. I am glad they caught him, but honestly there are thousands more out there that don't get caught and that is what's disturbing.