An American citizen presenting themselves at the border is entitled to enter the country. They can be questioned, searched, and required to quarantine as a public health measure. But as a general matter citizens cannot be denied entry into the country.
Here the government places a requirement on airlines to prevent citizens from presenting themselves at the border. You can’t even get tested in much of the world. And there are few places where you’re allowed to go enroute in order to stop for a test.