Being a "federation" has nothing to do with it. US States are expressly prohibited from closing their borders. That right to travel is near absolute in the US,

Being a "federation" has nothing to do with it. US States are expressly prohibited from closing their borders. That right to travel is near absolute in the US, even the Federal government can't prohibit it outside of very narrow cases that don't apply here. In the US, as a matter of Constitutional law, people have rights reserved to them that the government, State or Federal, cannot take away from them outside of narrow circumstances (like a felony conviction). Freedom of travel within the US happens to be one of those near absolute rights, like freedom of speech. Americans take this very seriously as a matter of culture; no court would allow it and there would be widespread civil disobedience if they tried.