The Constitutions Article 4 Privileges and Immunities Clause sets the bar. If NH or any state, were to block a non-resident from entering that state, they would be in violation of that clause. It is the part of the document that allows us to travel from state to state at will, except of course for tollsJust thinking out loud, I really have not formed an opinion. Aren't license plates a rather arbitrary way of doing this? What if you live in NH but go to NY to work or visit family? Why shouldn't you also be stopped when you return to NH?