There's a difference between weapons used to destroy enemy resources and weapons used to destroy enemy civilians and troops. Agent Orange is the former with a side effect of the latter, whereas something like mustard gas or the chemical weapons used in Syria are simply the latter. It's a siege weapon (like a catapult) instead of an attack weapon. I think the distinction is important. I agree that the use of Agent Orange was a travesty.
How are you defining "side effect"? It seems to be something like "effects which are not the stated goal", combined with "side effects do not define what a tool is". By that logic, we should just nuke all our enemies, since our enemies are in the blast radius. Deaths of everyone else nearby and fallout and all that stuff are just side effects, nukes are totally OK when they are used to attack, not siege.
I just don't think the US military realized the consequences of Agent Orange on the population when they came up with that terrible, terrible plan. Nobody is denying that Vietnam was a clusterfuck, but I believe Hanlon's razor applies.
Burning large areas of forest where you know people might be living (must be living, in the case of farms) can't be overlooked. Hidden from decision makers, but not overlooked.
