White Phosphorus (WP) can be used against equipment, even if the Equipment is being used by people (i.e. as long as the time being attacked is property NOT people WP is legal, even if People are harmed in the attack). If the target was property then WP is perfectly legal, if the target was people then it is NOT. I hate to say this but the target seems to be property, and that is a legitimate use of WP.
Now an exception exist if excess injuries to people over what property is being destroyed (i.e. the injuries to people out weigh the property damaged, but the burden is on the person saying that is the case).
A second exception exist if the purpose is to retaliate for excess use of WP by the other side. I notice the US did NOT even bring this up, for if that is the case the US will have to show its use was legal while the insurgent's use was illegal.
As to being in Cache, that means nothing, it could be being held for future use in a legal way, (i.e. to create Smoke or destroy things by Fire) or kept for retaliatory purposes, much like how the US kept Gas bombs ready for use during WWII, to be used if the Germans used Gas first (Yes, I know the US had decided to use Gas on Japan in any invasion of Japan, but that was between two countries that had NEVER RATIFIED the International Treaty on the use of Gas, yes the US and Japan did not RATIFY the treaty banning the use of Gas till long after WWII, in fact the US did not sign the treaty till the 1975 and Japan only Ratified the Treaty in 1970).
For more on US and Japan agreeing to the Ban of Chemical Warfare see:
http://www.sipri.org/contents/cbwarfare/cbw_research_doc/cbw_historical/cbw-hist-geneva-parties.htmlNow to the first question, where are these insurgents getting WP from? Unlike regular munitions WP have to be carefully stored do to the fact they are a semi-liquid and must be stored standing straight up, so the semi-liquid stays equally displaced within the shell. If stored on their side, the WP will slowly flow to the lower side and when fired from a cannon will tend to go off target in the direction of where the WP flowed to in the shell (and that can be anywhere, as the shell is loaded flat into a Cannon or mortar and then fired they is no way to predict which side will be the side with more WP thus the shell will land almost anywhere).
Thus WP shells are very particular to deal with, most people will avoid using them, but if used as intended, they are very effective. But given the needs to be carefully handled, and guerrilla generally inability to provide such careful handling, why would the insurgents have them? The only answer is the shells are either un-exploded munitions (duds) which the insurgents modified for their own use OR these shells were supplied as part of a bigger shipment and then left in the Caches for no one could figure out how to use them and thus the shells just sat in the Caches.
My point is weapons in a Cache proves nothing, the issue is how the weapons are used, and if used against Equipment, even if people are harmed, perfectly legal for that is how the US is claiming such Weapons are being used.