The DP free inhabitants do not have any legal rights in America. If they are born in America, they have the American citizenship and need to follow the standard laws just like anyone else. However, the US constitution does say the free inhabitants of different states should be treated equally in any state they visit.
The article was proposed to secure the independence and the sovereignty of the different states. The free inhabitants cannot consider themselves unaccountable for all their actions while claiming the security of a free citizen. Placing an argument over an old document is futile, especially, if the entire meaning of the document is unclear.