Every body is protecting their assest on the ground, for the U.S and coalition it is the Kurds, for Russia it is the SAA/Hezbollah and it's other allied militias, for Turkey it is the FSA and other Turkmen groups etc. So it's normal for the collation to draw a red line for Assad/Iran backed militias/army. I believe they got the message.
Good question which many people overlook and don't even pay attention to.
The truth is that Turkey's case is special and different(it has always been the case). Turkey is a MAJOR NATO ALLY/MEMBER and a major E.U Partner(for example they have free market trade deal/customs union with the E.U which very few non E.U countries have) . This fact alone changes EVERYTHING. So there are things Western powers will tolerate when it comes to a NATO ally like Turkey which they won't for any other Middle Eastern/Muslim country that isn't part of NATO like say Iran, Iraq, Egypt etc. It's also for similar reason western powers turned a blind eye and didn't intervene when Turkey invaded Cyprus. Had it been say Iran or Saddam's Iraq who tried that, then they would have been crushed with no questions asked.
So, I don't believe there will be any such clashes between the U/S/Coalition forces and Turkey. They might have some disagreements and conflict of interests in this aspect, but they won't fight each other. They will still come to some sort of agreement/negotiated settlement/concessions. Which I hope will be the case soon enough. Since it's inconceivable that NATO will fight one of its own member. This might tarnish the reputation of the organization and set a bad precedent, which I'm sure both sides are aware of this fact.