I find it astounding that in this day and age people don't understand that not all nations are the same, and that a [semi or once]sovereign state is not the same as a nation.
The United States is a Federal Republic consisting of 50 sovereign states, and eleven "insular territories" aka war prizes and other scraps of less than sovereign lands.
Some nations are confederacies, like Canada and Switzerland. Then there's the "United Kingdom" which is not the same as England, either.
Since, in theory, a US-flagged vessel literally "shows the flag" and is entitled to the full protection of the US government, including the use of military force, it only makes sense that non-citizens need not apply. If they want the protection of the US government--they can apply for citizenship here. Your choice, flag it wherever you feel protected.
There is very little federal motor vehicle regulation in the US. There is no federal motor vehicle registration in the US. The only federal ("US") aspect for pleasure craft, is that federal documentation is available to them, if they are at least 51% owned by a US citizen or business. Corporations must follow the same standard, 51% ownership by US citizens or they can't document the vessel here.
This is AFAIK
for two limited purposes only:
First, to establish clear and definite title for the highly portable goods, to discourage theft and fraud. That's fairly recent, based on the HIN which has only been required since 1972. Second, federal documentation serves to maintain a database of available vessels and owners for military conscript. That's right, there are many old and disrespected legacies in the laws of all nations, and ours includes provisions to find out where there are boats, so that if we need to pull of a domestic Dunkirk evacuation, or send George Washington's Ghost across the Delaware again, someone will be able to pick the right boats off a handy list. (Filed in a cabinet under "A" for "Armageddon Plans" one hopes.)
The "United Sovereign States of America" is, despite the efforts of many wrong-minded leaders to the contrary, still a federal republic, and by law it is supposed to remain that way. With 50 mainly sovereign states, each with very different laws in them. That way if one or two screw up, we can vote with our feet and move to someplace else that's more reasonable.
But if you want to talk about states-versus-nations...Just look at Germany and Italy, each less than half the age of the US, each built from many competing states. Tell a Sicilian that you're a Florentine about to marry his daughter, and you may have have one heck of a spat on your hands. Italian? Yeah, that's a good one. American? Like apple pie, not one apple.