Again I say, parents should not police their kids, they should give their kids and example to look up to. If you provide a bad example, or no example at all, than no amount of policing will stop the kid from doing bad things. But if you show them how to be a good person through your own actions, then no amount of bad influence will turn them astray.
See the point? Whats needed is not a lack of negative influence, its an adundance of positive.
I like the idea, but it's somewhat flawed. I know any number of people (not least my brother) who have had an exemplary example from parents, but have been led astray by bad influences. It happens, whether you like it or not. And that's why parents need to police the activities of their chilren.