When did "WMD" go from describing something intended to kill thousands or millions of people to an IED? Both are bad, but it seems like a dilution to make "WMD" effectively synonymous with "bomb".
The 'mass' part reflects the principle of damage radiating outward from the origin and affecting whoever/whatever happens to be in proximity, as opposed to the more targeted destruction caused by a bullet or handheld weapon. Of course you could kill multiple people with a handheld weapon or (rarely) hit several people with a single bullet, but in general such attacks involve one-on-one interactions even if there is a series of them in rapid order, as with automatic gunfire. hitting multiple individuals from a single shot or manual attack is considered a fluke.
See notes on http://www.law.cornell.edu/uscode/text/18/2332a and http://www.law.cornell.edu/uscode/text/18/921
Note that the law is not necessarily drafted to reflect the actual probability of injury, but to dissuade any use of the method; explosive, chemical, biological and radioactive attacks are by nature indiscriminate. That's considered much worse than an attack on a particular individual (you could launch an indiscriminate attack by, say, firing a gun into a crowd. But that lack of care about who you hit would itself attract a higher sentence). Furthermore, it radiates temporally; if you plant a device and then drop dead of a heart attack, the device remains dangerous even though you are no longer capable of forming criminal intent. It presents risk from when it's emplaced to when it has finished releasing, a period which could last years. In similar fashion, using a bomb or poison to kill someone automatically makes the offense into Murder in the First Degree, negating defenses like 'crime of passion' or 'heat of the moment'.
http://www.schneier.com/blog/archives/2009/04/definition_of_...
Here's the legal code which describes a "destructive device" (passed in 1968 and amended several times):
http://www.law.cornell.edu/uscode/text/18/921
Note that it is so broadly written that it has to specifically exclude antique muzzle-loading rifles.
Is a gun a weapon of mass destruction? Why not?
I think an assault rifle would've done a lot more damage than the pressure cooker.