There is another possibility, though, that I haven't seen discussed on FR (I know, I lay my self open to a good ol'flaming, but, please, just send the links if I'm wrong. :-) ) but has been discussed elsewhere on the net.
Here is one link that contains further links to other sites with similar analysis.
It's a long piece, especially if one wants to follow all the leads, so here is the abstracted version:
Fuel-air ammunition contains chemicals (eg ethylene oxide, propylene oxide) that if unexploded causes symptoms much like nerve gas agents. However, FA-munition or thermobaric bombs are not classified as CW-weapons and are not prohibited.
The hypocrisy would of course be astounding (but it is already), but a "dud" fuel-air bomb would not be considered a war crime. Would it be considered a "red line"?
Enquiring minds (ie no MSM-journos) like to know.