Agent Orange is not relatively harmless to people. It still maims people today in vietnam. I would never take a US claim of why they do things at face value. If we can think up collateral reasons for such uses, you can be sure they are thinking them up also. It seems on the face of it that as far as results were concerned the devastation of Agent Orange as a poison has been more thorough than as a defoliant, a process that was not terribly effective.
The US used gas in Vietnam also, in a tactical role. I think in places like the tunnels. CS gas can kill people in a confined space.
The military also assisted in The gassings at Waco, which was not just a violation at the chemical weapons level, but the restriction at the time against the army being used in civilian maters.
The US also was slow in giving up chemical weapons, and has not taken them out of the arsenals yet, though a plan is in place. While their continued connection with chemical weapons is a bad visual, one can have some sympathy as the us plays in the major leagues, and putting any tool out of use is not always wise. One does trust the US not to lead with chemical weapons as a major element in their warmaking.
On the other hand, the US has so many military capacities, that their ability to inflict misery in retaliation is not particularly limited to any one tool. One of the bad things about gas is the long term chronic pain it causes to survivors. Well welcome to Hiroshima and Nagasaki; or Agent Orange; or depleted uranium dust. Or the US doesn't use conventional terror tactics, but they can calibrate the use of drones in Pakistan or Yemen in such a way as to regress whole populations. A lot more vicious than the odd suicide bomb. So the US can take the high ground on something like chemical warfare when it suits them, and the toolbox is full of so many other tools.