i've argued that the turks were in fact their primary target, and that what you see open up in this region around this period is a proxy war between the saudis and the turks for control over syria. that is something that a lot of the independent media misunderstood, in either seeing these jihadist groups as interchangeable or misinterpreting the turks as the hegemon. at this stage, we can surely all concede that these groups were in open conflict with each other from the start, and the turkish-backed rebels and saudi-backed rebels were never operating with a common set of goals. but, perhaps the severity of the potential consequences of this were never fully understood. what would have happened if isis had defeated the proxies and moved directly into turkey? one would not expect the turks to have much difficulty in defeating the saudis.
i think this is where the american support for the kurds comes in. america's master plan is not anything specific, but hegemony over the region; assad was once useful to them, and may, in the future, become useful to them again. a part of maintaining that hegemony lies in creating chaos (being the great satan), but a part of it lies in keeping the turks and arabs in a careful suspension - played off against each other, but without allowing them to come to blows.
the kurds, at this time, were a useful buffer state to keep the saudis and turks from coming to direct blows.