So long as the foundation and the official "owners" of the kernel are US based, then the real answer is "because it's the law". Despite the fact the kernel is maintained and used throughout the globe, other countries' laws are entirely irrelevant, but people who employed in a country are typically held to its laws.

The real mistake was having a registered company in the US that they're unable to realistically move abroad.

In a world with sense, someone vaguely accountable in a new country will fork the kernel, that just becomes the de facto new kernel, doesn't seem likely. We can only wait and see.