The US knows perfectly well that Daa’sh poses no danger to them whatsoever; even Israel, right next door, does not feel any threat from Daa’sh, though the Jews are always the first ones to scream in terror if so much as a firecracker pops in an empty field nearby.
The “threat’ of Daa’sh to US interests is like the threat of the Sandinistas in Nicaragua in the 1980s. Which is to say, it is no threat at all, but on the contrary, a useful pretext for military intervention.
The expansionist rhetoric of Daa’sh is particularly helpful for US interests, insofar as it will allow them to justify military action in any and all of the lands Daa’sh hopes to encompass. Indeed, the director of the CIA is already saying that Daa’sh will provide the US with opportunities for military intervention all across the Middle East and Africa for the next 30 years.
30 years of destruction in the Muslim world, for the US, is definitely the same as winning.