Yes, because the US's image is tarnished with the world by this point. Maybe if we hadn't already pissed off every part of the world for one reason or another, the US might be able to play that card.
Fact is, the past five years have collectively been a foreign relations disaster. By this point, Syria couldve shot the helicopters, because, honestly, what's the US going to do in response?
Nothing. We can't afford to launch a full scale attack on another country. If we did label Syria as a country harboring terrorists and imposed embargoes, who would be on our side for it to make a difference?