The term “destructive device” means—
(A) any explosive, incendiary, or poison gas—
(iii) rocket having a propellant charge of more than four ounces,
(iv) missile having an explosive or incendiary charge of more than one-quarter ounce,
(v) mine, or
(vi) device similar to any of the devices described in the preceding clauses;
So, anything that you can throw that contains more than a quarter ounce of explosive is a WMD. Eventually, the terrorism label will be so over-used as to be essentially meaningless. This is a common form of categorical inflation. If you have laws that criminalize everyone, the concept of crime becomes meaningless. If everyone is declared a hero, then the concept of heroism is meaningless.
Defining a quarter ounce of explosive as a WMD makes the concept of WMDs into a joke. I mean, even a bigger joke than what Bush made out of it when he used it to fabricate an excuse to start a war with Iraq.