The US takes care of it's assets, it doesn't consider anyone it's friend or ally. That's actually quite common for a superpower, and ever superpowers before it has done the same. It will go to war when it feels that it's assets are threatened, but it will instantly leave you, if it feels that either you're a lost cause, or it has no use for you. A simple and recent example is the so called "Arab spring"; The US sided with Bahrain, who crushed innocent protesters, while it abandoned Mubarak, because he was a lost cause. Currently, they looking to replace Mubarak (and his successor, Morsi) with someone who's pro-US (Gen. Sisi).
No superpower acts without making sure it has something to gain, it's why the US controls all the worlds oceans and shipping routes.
Please don't think I'm bashing the US, because I'm not. It's doing what it has to, in order to stay on top, no one can argue against that.