There's basically only one reason not to register if you're a sex offender, and it isn't a morally decent reason.
There's alot more then one reason:
Trying to get a job without saying you were convicted
Thinking you did it but doing it incorrectly (This happened to someone i know, but he was 100% scum and I delighted when he had to explain himself to his parole officer and the judge)
Straight up forgetting to do it
None of those reasons are morally indecent (the first one maybe border line), Plus in many states you can be a registered sex offender for getting drunk in college and streaking, that's ****ed up imho.
I bet Son might have done this to get his prison stripes (so he won't have to be someone's bitch), which is odd because he too is a sex offender, in the worse kind of way too... and he's asian, probably the weakest (in numbers) ethnic group in american prisons.