Weapon of Mass Destruction: "Nuclear, chemical, and biological weapons." (1)
Nonlethal Weapon: "Chemical agent that can incapacitate but which, by themselves, are not intended to cause death. Examples are vomiting agents and psychochemicals such as BZ and LSD." (2)
In 1992, the United Nations agreed to the text of the Chemical Weapons Convention (3), a resolution prohibiting the development, production, stockpiling, and use of chemical weapons. The treaty went into effect in 1997 (4), although as of this WU not all of the 150 member countries of the UN have signed on. A provision is made to allow nonlethal weapons (riot-control agents) for purposes of "law enforcement", but specifically states that "each state party undertakes not to use riot-control agents as a method of warfare." In essence, it's perfectly okay to use agents such as tear gas on your own citizens, but it is a violation of international law to use nonlethal weapons during wartime. The Organization for the Prohibition of Chemical Weapons, paid for by dues from member countries (including the US) oversees and inspects countries to ensure their cooperation. As of 2000 their annual expenditure neared $60 million. (5)
This presents a severe restraint upon nation states, especially in situations that may involve a significant number of hostile civilians. Because they are chemical weapons (and therefore weapons of mass destruction), nonlethal weapons such as malodorants (bad smells) are on the same level as a hydrogen bomb. This forces nations at war to utilize significantly more lethal means to achieve their ends.
Sources:
(1) NT: WMD 411
http://www.nti.org/f_wmd411/f1a1.html
(2) New Mexico Weapons of Mass Destruction and Terrorism Web Site
http://www.wmd-nm.org/members/office/glossary/index.asp?where=n
(3) CHEMICAL WEAPONS CONVENTION
http://www.un.org/Depts/dda/WMD/cwc/
(4) Tear Gas May Be Banned in Battle
http://www.foxnews.com/story/0,2933,81564,00.html
(5) Chemica Weapons Ban May Suffer for Lack of Dues From Treaty's Parties
http://www.greencrossinternational.net/DigitalForum/digiforum/articles/article2001/chemicalweapons.html
Ventura County Star: Showdown With Iraq
http://www.insidevc.com/vcs/showdown_with_iraq/article/0,1375,VCS_9220_1843371,00.html
Troops train to use nonlethal weapon to control crowds, reduce civilian deaths
http://www.estripes.com/article.asp?section=&article=12421&archive=true