I hate to be on the side of sex offenders but it is literally unconstitutional in every sense based on the ex post facto clause of the US Constitution, where the government may not retroactively levy penalties or liabilities. Does it suck as applied here? Hell yeah, but arguably if they can cross this line today, what line will be crossed tomorrow? The ex post facto clause along with prohibitions on bills of attainder have a strong respected history in American Jurisprudence and arguably makeup one of the first major divisions between English and American Law (aka we fought a revolutionary war over it).