Would Apple's shareholders put up with Apple not being able to sell into the U.S. market?

We're getting into "kill the company to make a point" territory, which is probably not realistic however we might imagine they do it.

Whatever leverage Apple has over the federal government will come from the millions of citizens who are customers, shareholders, developers, fans, employees, etc. They can activate those people just as well from Cupertino as they can from their own island.