I don't think the US cares who comes out on top as long as we can maintian some influence over the winner and protect Israel at the same time.
The US should care.
Iran has the bomb.
Then wouldn't it be foolish to give them so much territory and oil to boot .
Don't forget that sooner or later Iran will be forced to dispach its military and air force to get seriously involved, which with any doubt would be more than a match for ISIS and swiftly reclaim lost territory.
There is no doubt the Iranian military is already advising the Iraqi army with strategy and intelligence and the terrorist group Hezbollah is actively fighting and suffering losses in its battle against ISIS.
Oil supplies are not a concern for the world as evidenced in the price of oil worldwide.
If the Iranian block can't contain ISIS, then is the time to step in with deadly force, however, at the present time let's not do the dirty work for Iran, a major supporter of terrorism.