This is also my kind of take. But not just sports leagues; it's mainly the member teams in the leagues. Many of them are probably into some kind of partnership or collaboration with bookmakers.
That's probably what happened and I think they are now feeling the pressure.
So it only shows that the league has connections with bookmakers, and it raises the possibility that match-fixing could be happening, doesn't it?
But then I realized Kalshi, and other prediction markets for this matter, are also paying these sports leagues for using their data or their games in the prediction markets. So why should these sports leagues be "extremely angry" when they're also benefitting from it?
Because sports leagues probably have partnerships with sportsbooks, not just bookmakers.
That's the biggest possible connection, so they might just be protecting their business partners.