There's nothing illegal about being a sovereign citizen and it's there right to be a sovereign citizen if they want to be.
A Sovereign Citizen basically doesn't think they have to follow the laws that other US citizens must follow and they also don't recognize the government or there laws.
Sovereign Citizens also do not recognize U.S. currency and maintain that they are "free of any legal obligations when it comes to law.
However the Sovereign Citizen can also be arrested anyway if they do not follow the laws that everyone else has to follow.
So being a sovereign citizen doesn't mean you cannot follow the law but it's not illegal to be a sovereign citizen.