You're forgetting to add why the US bombed Cambodia. The North Vietnamese were using the border areas with Vietnam as a refuge and a supply route for war materials.
Note that the US didn't bomb Cambodia until quite late in the war. That probably helped the North Vietnamese more than anything.
The bombing happened all over the country. A few weeks ago I was in Kompung Thom (right in the middle of Cambodia) and talked to quite a few people about this. The members of these communities were farmers, not combatants or VC sympathizers, and they didn't even have much awareness of why they were being bombed.
I also have to add that there is no reasonable "why" to excuse bombing largely civilian areas. Undoubtably the VC were crossing over the border -- the Cambodian side of the Vietnam-Cambodia border is ethnically and linguistically a mixture of Khmer and Vietnamese, and that border has been fluid up until recent history -- but still most people there would have been civilians.
I'll stick with "an area with many civilians" as my definition of a civilian area. These people weren't wealthy, and it's where they happened to own land and build houses. It becomes a choice between being homeless and impoverished, or staying where you are and taking the risk of being bombed.
If you commit to the idea "I will not bomb civilians", then you offer an enormous advantage to your adversary, who will therefore proceed to make sure civilians are well-distributed in any area of military operation.
Note that the US didn't bomb Cambodia until quite late in the war. That probably helped the North Vietnamese more than anything.