There is a social responsibility to stop abuse- the fact that nobody said or did anything about clearly inappropriate behaviour is awful, and at least some of the parents should have noted this unless they too were in awe of Michael, and if so then they do also carry some burden in my opinion.
When you work with children (no matter how famous you are), there are rules in place to ensure the protection of those children and those working with the children. In the UK all adults must be CRB checked and must not be alone with a child in a one-on-one situation unless there is some otherwise listed circumstance, and you must be in-sight of another adult. This is to both protect the child and to protect adults against accusations leading from any misunderstandings. Was there no child protection in place at Michaels ranch? Who's job was it to ensure that it should have been? It was a complete social failure.