Because the registration requirements themselves do not constitute punishment, it is of no consequence whether or not one has committed an offense that is 'sexual' in nature before being required to register.
This is a brilliant strategy for future legislation and criminal justice, and I would commend it to everyone. As long as the "punishment" is regulatory, we can justify any such rules without need to be subject to the 8th amendment's bar on cruel and unusual punishment. Here's how it might work. We decide that Wall Street executives have abused the financial system for too long, so we pass a law that any such executive cannot live within 100 miles of any sentient being. Now, the brilliance of this strategy is that, since this is not punishment, we do not have to worry about convicting the executive of anything. That would be necessary for punishment, sure, but not for merely regulating them. But if we really wanted some excuse, we could regulate their living arrangements pursuant to some other law, say the law against jaywalking, and say that any jaywalker who was a Wall Street executive was subject to this regulation. Then all we have to do is convict them on the lesser jaywalking charge in order to subject them to these other regulations.
Antonin Scalia would certainly not object to this since he is on record as saying that torture of terrorist suspects does not violate the ban on cruel and unusual punishment since they have not been tried or convicted of anything and are thus not being punished. Similarly, then Scalia should endorse the government employing thugs to break into the home of every Wall Street executive and beating the crap of him/her, his/her family and pets. You might think this is cruel and unusual, but it certainly isn't punishment because we're doing it just for the hell of it.
Here's another strategy for use on the executives. We don't put them in prison per se, we just regulate their movements so that they are required to live within a very small cell in a maximum security penitentiary. You might say, putting them in prison is punishing them, but no, we're only regulating their movements and living arrangements. That's not punishment and is not then subject to these restrictions on punishment. Since it's not punishment, the restrictions need not be relevant to anything for which the executive has been convicted. And, if Scalia is right, it's perfectly constitutional to have the guards beat the crap out of them every day with a baton as long as they're not being punished for anything, but the guards are just having fun and blowing off steam.
I'm just wondering if the legislature could pass some regulations regarding the movements of certain Supreme Court Justices--perhaps requiring that they never come within 1000 feet of a legal case. I'm fairly certain none of them (Scalia and the majority of the GA Supreme Court) has ever been within 1000 feet of common human decency.