For me it's not about a stranger talking to my child, in usually ok with that, it's about a stranger thinking they have the right to discipline my child. That's not right. Only two people have that, myself and DH. We have agreed how to do this and back each other up, anyone else including close relatives will be just confusing.
With a stranger it's even worse. They don't know me or my child or what he's like. If you have a problem with my parenting, talk to me or forever hold your peace.