The NLF were not using Cambodia to the extent that the Nixon administration described it as.
There were NLF medical camps in Cambodia, however-- there were not major divisional units based in Cambodia.
In a previous post you admit that the North Vietnamese were using the area to stage attacks on South Vietnam. Moreover, the Ho Chi Minh trail, by which the Communist supplied their forces in South Vietnam, ran through the area. Those two facts were sufficient to justify sending our forces there.