Now, to be clear: BZ is a real chemical incapacitant. The United States, the United Kingdom, and others stockpiled it. U.S. scientists discovered BZ in 1951, producing it as a byproduct of peaceful chemical production (though not in a pure or isolated form). Iraq did research on BZ, including importing a sample from Egypt. There is no evidence that Syria has a BZ program, which is probably why the National Security Council released a statement describing the allegations outlined in the cable as "not consistent with what we believe to be true about the Syrian chemical weapons program."
Incapacitating agents, by the way, are not what one usually thinks of as "chemical weapons" -- nerve gas and the like. BZ and other chemical incapacitants arise out of the same deep well of craziness that led the government to develop LSD. The Chemical Weapons Convention largely dried up the crazy. We haven't seen something spectacularly stupid since 1994, when the predecessor of the Air Force Research Laboratory was considering a proposal to develop a chemical agent that would "cause homosexual behavior" in the hopes of adversely affecting "discipline and morale." Incapacitating agents like BZ are controlled under Schedule 2 of the Chemical Weapons Convention. Schedule 1 is where are all the interesting things like mustard gas, sarin, and VX are listed. States do not even need to declare Schedule 2 chemicals if they are present in "low concentrations." In 2004, the states party to the CWC had a very boring, technical debate about when states should declare incidental production of BZ and two other Schedule 2 chemicals.
I don't want to minimize the dangers of incapacitating agents. As an impressionable research assistant at CSIS, I participated in a study on non-lethal weapons which made very clear that all "non-lethal" weapons can still kill some people, who react in all sorts of unique and individual ways. (There was a proposal for a sticky foam that would harden into something like concrete to immobilize rioters. It seemed like a really good idea -- for about 15 seconds, until someone asked how Han Solo breathed in carbonite.) At a simpler level, you can imagine the carnage if someone fired "just" tear gas into a nursing home.
But the allegation that Syria has used a chemical weapon isn't really about the dangers of BZ or incapacitating agents. It is really an argument about the Syrian government violating a norm that places them outside the family of nations and compels us to intervene. Sure you can torture people or shell their villages, but poison gas?
Not surprisingly, one finds plenty of evidence-less allegations of chemical weapons use by groups seeking to encourage foreign intervention. In July 1995, for example, the Bosnians alleged that Yugoslav forces gassed them with BZ -- at a time when the United States was still following Colin Powell's suggestion to let Bosnia burn. (A few weeks later Clinton would authorize an air campaign, Operation Deliberate Force, a decision Powell now says was the correct one.) Kosovar Albanians made similar allegations of BZ use in April 1999 during Operation Allied Force. There is not a lot of evidence for either claim and, to my knowledge, the International Criminal Tribunal for the former Yugoslavia has never charged any Serbs with regards to this. Perhaps it's too cynical to put allegations of chemical weapons use in the bin with bayonetting babies or dumping them out of incubators, but truth is the first casualty and all that.