In the US, the States are explicitly prohibited from closing their borders as a matter of Constitutional law. For different reasons, neither the States nor the Federal government are allowed to prohibit free travel between the States. This has been to the Supreme Court many times, it is mostly settled law at this point.

The right to travel within the US, like free speech, is near absolute. The due process hurdles to temporarily remove that right from an individual are very high; you can't do it with an edict nor de facto travel restrictions by abusing regulatory power (which has also been tested in the US Supreme Court).

Australia has significantly weaker individual freedoms than the US. This is one of those cases where those differences become apparent. Everyone asserting that the US should restrict travel like everyone else is ignoring that it is expressly illegal for the government to mandate such at thing in the US. No one in the US government is interested in dealing with the backlash such an attempt would elicit.