In general I agree. It would be nice, in the long run, for the US and Iran to work together.
But that isn't the case today. And it isn't likely to be the case in the near future.
And, currently, SA and Iran are at loggerheads. The choices are pretty much to choose a side or step back and watch the carnage. The latter choice diminishes the US influence in the region over time (which may well happen anyway). Iran also has influence in Iraq, complicating things there (to say the least) as well as Lebanon and Syria.
So the political calculation pushes the US towards the Saudis. Yes, they are a vile regime. And their abuses should NOT be ignored. But find a major power in the region that is NOT a vile regime.