Why do you think the US supports Saudi Arabia? It does so especially because it's so weak, to secure Arabia's oil and to ensure they act as an appropriate counterbalance to Iran. It used to be Iraq but well... thanks to the Americans, not anymore. Unless the Americans go into isolationism, they'll probably defend or support Saudi Arabia in a war against Iran, because having an American-opposed nation like Iran dominating one of the biggest oil reserves on the planet is a big no-no. So if Donald Trump was trying to stop terrorism he would include Saudi Arabia, but as he, and naturally like no other American leader wants to insult it.
It's a sad day that the US is so meek with the Saudi's, helping them stay in power and ignoring their atrocities. It's sadder because the Saudi's use America's lack of courage with them to become more and more bolder, as we can see from their invasion of Yemen and blatant support of terrorists.