Don't most of the writers for TNG admit the 'Roddenberry box' is what forced them to be more creative with their stories? I don't think the lack of crime on Earth is meant to be an absolute statement. Crime must exist, but it's unexpected to the point that most people feel safe enough to leave their door unlocked. There's obviously crimes of passion, and the person here and there who tries to get around trade regulations and such. But, in a world where scarcity is eliminated, it's easy to imagine there's very little crime.