Why is a pipe bomb that kills 3 people called a Weapon of Mass Destruction, but a gun that kills 27 is not?

Pipe bomb is often bad because there is little to no reason to use it for self-defense unless you're in a desperate situation where you are cornered by gunmen, which I would doubt it happens. That's the reason why they are not sold in retail stores unlike guns. Pipe bomb can easily kill people to those who came close to it whereby a gun does not.

Mass destruction refers to buildings, sidewalks, mail boxes, items that you saw at the bomb site in Boston. As well as the human loss.

It depends on who is holding the gun, at lest according to the right. If a Muslim it holding it then it is an act of terrorism inspired by a "hate filled religion". If on the other hand it is a Christian then it is a random act of violence committed by a lone deranged person.

a pipe bomb is not considered a defensive weapon, whereas a gun often is

"Nuclear weapons or nuclear-weapons-usable material or any sub-systems or components or any research, development, support or manufacturing facilities relating to [nuclear weapons].

Chemical and biological weapons and all stocks of agents and all related subsystems and components and all research,development,support and manufacturing facilities.

Ballistic missiles with a range greater than 150 kilometres and related major parts, and repair and production facilities."[24]

Because the ONLY purpose for a pipe bomb is to kill people... period.

If all you have is a hammer everything looks like a nail, so what if all you have is a gun...

What about a pressure cooker, what would you call that?

Something to cook your chicken in? Hmmmm