In theory, these registries are list of every sex offender in the state, with the his house location and other pertinent facts to help people avoid exposing themselves to such people. Megan's Law requires sex offenders to register and update law enforcement every time they change location.
This is not the result. You have to really hate the guy to make him suffer for the rest of his life, even when his prison time is up. This sounds perfect for a serial rapist or pedophile, but its not such a great idea if something as trivial as public indecency or streaking can put you on the registry right alongside them.
So you have a guy who committed a crime. Will putting him on a list make it better? wouldn't this only make people shun him, keep him from getting a job, and making friends? Just for a crime that he may have committed over 15 years ago as a adolescent? On a side note, one fifth of all rapes are committed by a juvenile.
And how effective is it? Even if you know about a registered sex offender in the neighborhood, what's to stop him from doing it again? And that's not taking into consideration that 95 percent of all cases are from someone the victim had already knew? And if he was really going to do it again, would the fact that he is on a list really going to stop him?