You understand that 'donors in Saudi Arabia' provoding money is rather different from 'the Saudis' (which in normal parlance means the Saudi government) funding ISIS/terrorism? That situation is in contrast to Iranian government funding of terror.
Add to this that Saudi is aligned with US foreign policy goals while Iran opposes them and it's not hard to see why the US supports Saudi.
Suadi Arabia isn't a democracy for starters. Their oil is owned by the ruling elite and their wealthy friends, as are almost all businesses and some of their wealth is going to Sunni terrorist organisations. Isis isn't rattling tins on street corners, they were getting large chunks of money from within Saudi. The people with this sort of money are the ones running the show, the Saudi 'government' is doing little to stop it. They are paying lip service to US foreign policy.