Holder called it a WMD in the charge papers instead of an IED in an effort to exaggerate the severity of the crime and invoke certain emotional responses from the media and, ergo, the public. It invokes vile visions of NBC weaponry, as opposed to the simple but effective claymore that it really is. The intent is to ultimately lead to increasing the chances of conviction and resulting in a stiffer sentence. Lawyers do it all the time. It's called puffery, and Holder is the top lawyer of the country right now. I don't think there's any conspiracy going on here other than an over-indulgent use of a thesaurus.