The invention of “Khorasan” as a faction in Syria was meant to disguise that the US attacks in Syria were targeting not just ISIS, but also Jabhat al-Nusra, the local al-Qaeda affiliate whose members were attacked under the guise of targeting Khorasan.
The attacks were unpopular among the “moderate” rebels the US is always talking up, who noted Nusra is a close ally of theirs. Or at least, they were a close ally.
With the US attacking Nusra, the pro-US rebels are now facing retaliation from Nusra fighters, and it’s yet another enemy they can’t actually beat, leading them to push the US for even more aid.
The US seems to have given up on the existing moderate rebels, and is preparing to create a new faction in the years to come, but the attacks are reflecting both the unsavory nature of the existing US allied rebels, and how little thought went into expanding the war against Nusra.