The money quote is in the lead paragraph (gotta love the British press sometimes):
"It was used as an incendiary weapon against enemy combatants," spokesman Lt Col Barry Venable told the BBC.
From this we know that the 30% or so Americans (and higher percentage of bloggers, it seems) who will defend absolutely anything that the U.S. military does, no matter how heinous, are now required to lean pretty much exclusively on the "White Phosphorous is not a banned munition" argument.
It is true that White Phosphorous is not a banned munition. That's because its primary purpose - illumination - is specifically allowed under Protocol III of the Geneva Convention, even if there may be an incidental effect of fire, burns, etc.
HOWEVER, Protocol III also states the following:
It is prohibited in all circumstances to make any military objective located within a concentration of civilians the object of attack by air-delivered incendiary weapons.
This ends the argument. The U.S. fired incendiary weapons into Fallujah, a densely populated city. It is illegal to fire incendiary weapons into a densely populated city. Period.
There are a lot of old teaching stories that deal with the human desire to argue. A man who fancies himself wise will come upon two men having an argument, and he'll explain the disagreement simply and clearly and in such a way as to moot the entire affair.
The men generally fall upon the "wise man" and kill him. What the wise man was not quite wise enough to realize is that the two mens' objective was to argue, not any higher goal.
This, at least in part, is the reason that you will not find any major newspaper that will print the portion of the Geneva convention I quote above. There's no angle in ending an argument definitively by pointing out the obvious.
NOTE: The U.S. has not ratified the portion of the Geneva convention that this is excerpted from. This makes the use of WP legal under US law. I would be overjoyed to see the White House mount this defense. It would be honest, and it would allow Americans to see what their government is really like. We have refused to sign this portion of the Geneva convention specifically so that we will be able to legally fire incendiary weapons into crowded cities, which activity of course has the entirely foreseeable consequence of burning to death many innocent men, women and children.