Laos and Cambodia weren't participants in the war but that doesn't really feel like it matters all that much. Whenever I talk about the bombings of Cambodia and Laos with Americans (who - liberals and conservatives alike feel they must always defend) I sometimes here "well we bombed cities in Germany and Japan in WW2 and no one talks about those being war crimes". But were they? I really don't know much about those bombings. My gut says yes they were also war crimes but we just accept them because they were combatant countries?
To add some additional information, the firebombings carried out by the US on Japan killed more people than a nuke and are often forgotten about in the face of the two nuclear detonations. The Japanese were looking for an out for any reason possible before the nukes were dropped and the US was decently aware of this fact which makes the dropping of the nukes all that more unjustifiable. I would argue that both are equally evil but that come Laos and Cambodia the technology advanced enough for this campaign to be more brutal just like how technology has advanced enough now for us to remotely drop bombs on poor people in the mountains of Afghanistan.