tension on kernel mailing lists continues to grow as a Linux Foundation board member finally replies with a "summary of the legal advice the kernel is operating under" re: enforcing US sanctions
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.
For the same reason Google has to abide by EU rules and regulations and VW has to abide by American laws and Disney has to edit their movies for China.
Maybe not, but they probably has to be very careful about taking money from anybody with Russian citizen ship. Also probably, now has to be careful about having people with Russian citizenship, ties to Russian businesses, or the Russian government in any leadership or critical infrastructure positions.