You assume these parents don't beat their children every chance they get... I've seen Christian parents complain about spanking their kid three times a day, and getting no results. It takes every drop of restraint to avoid telling them, "Maybe you should try a different strategy then, hmm??????"
Quitting smoking was easier.
I always wonder how important religion is to easing the parental burden. Once a child has fear of a supernatural entity that can torch you over and over for eternity, the parent has a 'Trump' card, so to speak. LOL.
When children are very small they have Santa Claus to keep them in line, but once they outgrow that lie they're given another, more persistent lie to keep them from being naughty. Doesn't always work, though...