Let's see, Iran is a fundamentalist Shi'ite theocracy which has used terrorism in the past. Saudi Arabia is a Wahabi (ultra-conservative Sunni) monarchy which by all accounts has a lot of ties to terrorism. We've had faction-based religious wars in Europe, it can happen in the Middle East. I shouldn't have to detail Shi'ite/Sunni relations in the past. I'm just saying that it wouldn't be unfounded for Saudi Arabia to be worried about Iran suddenly getting tired with 'the keeper of Mohammed's home' feeding The Great Satan (us). Maybe they don't want to be seen as an unable observer in the event of Israeli/Iranian war. You know Israel would target anyone giving assistance to it's enemies, which pretty much means all Arab countries in the vicinity.
I know I'm speculating, but I just mean to demonstrate it's not all as cut-and-dry as some would have us believe.