I'm curious - why is this a matter for the supreme court and not the legislative branch who usually make the rules on taxation?

Because Congress has abrogated its duty on this matter.

The list of Congress' abrogations could fill volumes (they could call it the Federal Register).

No, its made a choice clearly within its Constitutional power, you just don't like the choice. That's bot an abrogation of duty (there are lots of places you can make an argument for doing that, but this isn't one.)

Enforcement via FATCA was a last minute amendment to a 1,000 page bill. No one read it, and it was overly broad - meant to capture people in the US hiding cash offshore but instead captured people who live outside the US and legitimately have local bank accounts.