I think it's depressing how world governments get so caught up in stuff like chemical weapons. For some reason that became the line in the sand. Why are chemical weapons killing people worse than the Assad regime murdering around 2000 civilians in just 4 months during the violent crackdown against the protestors in 2011? Why wasn't that the line in the sand? You can talk about the pain and suffering felt by the people killed in the gas attack, but that's nothing compared to the butchery that we know for certain was carried out by Assad back then.
Same thing with the heart eating. There's thousands of Syrians getting bombed out of their houses every day by an oppressive government that took its crackdown too far, kids dying and being left homeless/orphans left and right, yet people focus on an FSA captain eating a dead man's heart and therefore everything they've heard about the Syrian government's crimes magically becomes "Western propaganda". It's just idiocy.
Well, I'll explain it like this: At the end of the Second World War, the Allied forces had enough small arms ammunition to kill every single human being alive, or at least could purchase the remaining amount without much hassle. And this caused no one consternation, rightly, due to the fact that it would be long, costly, and surely someone one stop it before long. WMDs solved this.
Chemical Weapons are fast, relatively easy to stockpile and use, and indiscriminate. They're much like mines, which have also seen a extensive push to eliminate. They're large-scale, destructive, easily used, and have effects that last a long time. Also, while it's one impossible thing to try to remove guns, it's a entirely different animal to attempt to prevent Chemical weapons, which haven't seen much use and have a rather specific requirement for materials and storage. Also, unlike regular fighting, in which someone can have a claim to legitimacy, Chemical weapons immediately throw out any hint of any decency.
Simply put, it's a pointlessly violent weapon that is relatively easy to track down and eliminate use of, with little political middle ground for people to hide in. You can justify, as Assad has done, murdering civilians as only killing some terrorists, and still have your backers back you, using Chemical weapons is a statement that you simply don't care. A gun is accurate, you can use it to kill terrorists and only terrorists, but Chemical is a guarentee, which even Russia can't justify.
The whole thing about taboo on chemicals weapons seems absurd to me after nobody influential said anything about Israel's use of white phosphorus... though I'm probably not as informed as I should be on that point.
I confess ignorance.
I actually meant to write about this, because it's an interesting question. I strongly doubt the US would want to intervene, but arguably they should all the same. Using the logic from my first point they ought to identify and strike any rebel chemical weapon stockpiles. That said, the US is unlikely to want to inflict the harm that strikes would cause against the rebels and it's questionable whether more strikes are likely (how many weapons could the rebels have seized?), reducing the humanitarian argument. More likely (IMO) would be attempts to identify rebels and groups who had used or facilitated the use of such weapons and attempts to prosecute them for war crimes. That may be enough to maintain the chemical weapon taboo, assuming the prosecutions were prompt and effective. That is a big assumption though, and would probably mean coordination between various groups to capture and bring to justice senior rebels, an action not without major consequences for the rebellion in itself.
As an aside, I have seen war crime prosecutions mentioned in another article recently; this interview with the rebel cannibal.Abu Sakkar is in a freshly pressed uniform, jauntily smoking a cigarette in a way that lends a slightly absurd air to the whole performance. He says he's willing to stand trial - but only if President Bashar Assad does too.
Thing is, striking chemical weapon stockpiles is only slightly less dangerous then attacking a nuclear fuel facility. The potential for spillage and contamination is great, and the potential for wannabe terrorists to steal some even greater.