Because as some have found out, US law kinda applies everywhere.

https://en.wikipedia.org/wiki/Raoul_Weil

The better question is "Why does Facebook think that being based in Switzerland will protect them?

It's been a while since Swiss banks were last immune to U.S. government pressure. Or even pressure from other countries. I can't bother to find the links now, but there were a couple of stories during Obama's time, about the US government arresting people based on their financial transactions in Switzerland.

Because the US can go after every single American company that's part of the association that runs it.