Kweassa, they might have also done it early on, I believe.
I recall Pappy Boyington counted 3-4 planes shot down on the ground as "kills" but when he moved to the USN they didn't count them, so he wasn't an "ace" in their book. [EDIT: I *think* he switched from the USAAC to the USN, which is why I think maybe the USAAC counted parked planes]
I've seen some kill markings that show normal planes and planes sitting on their gear on a line (as a profile sitting on the ground) so I guess they counted them, but made a distinction.