Seems pretty clear to me. Bombs are a weapon. They are intended to cause mass destruction. Therefore they are a weapon of mass destruction. What part of this is unclear?
Because the meaning of the term weapons of mass destruction is well established as primarily concerning N/B/C weapons and not low yield IEDs. The fact that they caused a lot of damage and chaos doesn't change how the term has traditionally been used. When that elderly man plowed through the Santa Monica Farmer's Market in his car killing ten and injuring 63 they didn't start calling his car a weapon of mass destruction (though I'll grant it's not designed to be a weapon, unlike an IED, but that's still not how we've used the term WMDs).
Okay, I got the basic thrust of that gist.