Well, there’s an interesting story which might be somewhat relevant to this question.
The Earl of Dundonald, Sir Thomas Cochrane, was a very interesting guy. He got a hold of a slow little brig named the Speedy and raised absolute hell among French and Spanish commerce in the Med, and became a very famous fellow indeed. His character and his exploits were the model for Patrick O’Brian’s first Aubrey/Maturin novel, Master and Commander.
After that, Cochrane got hisself a strange idea. Perhaps inspired by the occasional man-o-war magazine explosion which often damaged other ships nearby, he designed a series of boats which were absolutely loaded with nasty shit. One, an “explosion ship,” was planned to be loaded with powder and shrapnel, towed into an enemy harbor, and detonated. A variant of that design was to be loaded with rotting animal carcasses as well, to spread disease-carrying decaying flesh. Still another model was to carry sulphur, and be ignited in a harbor, causing suffocation to anyone nearby. Chemical, biological, and big-ass explosion warfare.
The Admiralty was abhorred. They made Cochrane take a vow of silence, and kept his ideas completely secret for decades. Why? Because the British realized that such an idea was even more dangerous if someone else used those weapons against them. I think that’s the nature of the taboo. One usually doesn’t want to unleash a new form of warfare when one cannot easily defend against it.
Addressing the broader question you ask, the problem as I see it is this: the United States and its allies have so far outstripped the rest of the world in military technology that is borders upon folly to try to compete. But people and nations will fight one another, so some bright boy decided to change the name of the game.
Prior to the fall of the Soviet Union, nations could make a choice as to which camp best suited its long-term objectives. That option doesn’t exist any longer (not until next year, anyway, when China’s mobile ICBM platforms become operational).
Asymmetric warfare of the style that has been recently visited upon the world is a logical alternative for people and nations that cannot compete on a standard military level. Furthermore, by disassociating themselves from any one nation, only the small group which perpetrated the act appears culpable. The truth of the matter is that al-Qaeda does serve the interests of many nations.
The near complete silence of the moderate Islamic world in the wake of the 9/11 events is telling, to me at least. The fact of the matter is that those tactics worked, spectacularly; while not overtly condoned, many nations appear to be silently assenting to such behavior.
Because it works, where nothing else does.
Remaining taboos? There are plenty. Mass sterilization, electromagnetic spectrum anti-personnel weapons, de-orbiting asteroids, you name it. If it can destroy nations, someone is contemplating it, that I can guarantee you.