China and the US may not have a common enemy, but we have a common need: oil from the Gulf region. And in order to peacefully obtain all that we need (at a price we can afford), we may be called on by the powers-that-be in that region to denounce our only real ally there.
I somehow don’t see us denouncing the Saud family any time soon. Especially when what would replace them in that country is far far worse.