This is somebody else’s fight. The US has no vital interest at stake.
Sometimes the excuse is not the reason for a particular action.
Perhaps President Trump wants something from the Saudies and this is just a way of getting it.
Trump got the pastor back from Turkey. Trump claims it had nothing to do with the muzzy journalist but they wouldn’t have BAM! turned him over so quickly otherwise. Now, what can we get SA to give up? It doesn’t matter the journalist is nothing to the USA but it makes the Saudis look bad on the world stage. Art of the deal. Strike while the iron is hot. Get it while you can.