As some of the comments have pointed out, the WMD definition is elastic. The Wikipedia listing is the usual starting point.
One framework boundary is that it used against non-combatants. Therefore, the US can nuke an SLBM launch platform (i.e., an enemy nuclear submarine carrying missiles) without it being a weapon of mass destruction because it's used against a military target. If the exact same device is detonated over a city it's a WMD.
Second is that there is no realistic shelter. Londoners were able to hide in the subways during the Blitz. This wouldn't have been true if the bombs had been chemical or biological. V2s were WMDs since they simply landed somewhere at random - no one could see or hear them, and there was no countermeasure for shooting them down.
Third is that it takes the form of a 'sucker punch' - you discover you're at 'war' after hostilities have commenced. Thus people attending a marathon have no idea someone intends to blow them to smithereens, whereas people in various parts of Israel are aware that rockets could be launched from the Gaza Strip at any time. Hamas and Hezbollah have made no secret of their intent to commit violence against people in Israel. While using rockets against civilians violates the rules of war and international conventions, they wouldn't be a WMD on this criteria taken alone.
The US government probably looks at intent when deciding what constitutes a WMD. While a device may be not kill anyone, the intent to promote panic in a crowded place and thus lead to consequential injuries or fatalities also makes it a WMD. Again, a kid playing with explosives in a drainage ditch has no intent of harming anything, the same construction in a theater doesn't have to explode, it only has to be discovered and incite panic.