This isn't how "responsibility to its shareholders" works.

While it's true activist shareholders often launch lawsuits over issues they don't like in the US, they are rarely successful and more often looking for a settlement involving a secondary, less significant issue.

US Courts have repeatedly given a large amount of freedom to management to do whatever they want in terms of "delivering shareholder value". They allow management so consider long term goals against next quarter (as they should).

Additionally there's an excellent precedent for this: Google withdrawing from the Chinese market. There are other precedents for threats like this: Both Facebook and Google threatened to withdraw from Australia in various forms during negotiations over payments for news.

(Not arguing this is a good idea, just that it's completely within the realm of what Apple could do).