Why doesnt the 8th admendment apply to megans law?

the U.S. Supreme court has stated "that a punishment must not by its severity be degrading to human dignity.Losing your job home and being put on a list where you are not allowed to move on with your life is the MOST DEGRADING THING that can happen to you.so again i ask how does the 8th amendment doesn't apply.AGAIN this is from a purely LEGAL POINT OF VIEW.