The US makes an abstract policy distinction between Iraq and Syria, and in Syria only attacks PISSI's logistical support, not its troops. This is why it has not provided close air support to the Kurds fighting to defend Kobani.

Meanwhile, the US wants Turkey to fight this battle, but Turkey would rather fight Assad that PISSI.

Close air support is the only sort of fighting that the US ought to do against PISSI. Once it starts attacking urban facilities, it causes civilian casualties.