He presumably didn't voluntarily put himself on the registry. He was caught and charged. Otherwise it wouldn't have been classified in any way, he wouldn't have been sentenced, and he wouldn't be on the registry.

The fact this experience apparently didn't communicate to him how fucked up this is is a sad indictment of punitive "justice" systems and should lead us to consider rehabilitative justice. This guy should still be doing the work to reform/rehab himself, not just be back out in society defending his actions.