Coalition forces can not fight aganst PKK, because they are still in conflict with militas inside of the Iraq(Baghdad and around). And I don't think that they want to get in danger.
According to the latest news, US sent a U-2 spy to the Northern Iraq border to watch PKK actions, but some of our citizens are foolishy in doubt that US is not watching PKK on the contrary they think US is watchin Turkish forces and that made me laugh

And also according to the some American sources, US may allow Turkey to make an operation in Northern Iraq but not an expanded one. It would be an operation including Air Bombing on some targets and so on, but not a large one.