You would be a fool to think that the war aganist ISIS is merely a battle of good vs evil. The world does not work like that.It's actually well documented that Turkey is helping rebels in Syria, even going to the extent of helping rebel groups like Al Nusra, Al Qaeda affiliates and ISIS... In return, they have been able to obtain enormous profits from oil revenues and contain a hostile Kurdish population... In fact, Turkish border patrols and intelligence, allowed the mastermind of the recent Paris attacks to cross the border of Turkey and Syria on multiple occasions, but yet, they see a Russian front line bomber as a bigger threat, the question is why? Because Turkey has everything to gain from instability in Syria and Iraq... Those things I mentioned are just the tip of the ice berg... Turkey does not belong in NATO...
Turkey, like any other nation, will always act to what it considers it's best interests. Supporting NATO in the battle aganist ISIS is probably not in Turkey's best interests so why they should they do it?
The government of a nation should always act in the best interests of it's nation.