It is fairly astounding that you did not use the "NOT" in the correct fashion as we discussed that at length in
/t/14752/1
That thread also referenced the help on filters so that the keywords for the existence or non-existence of fields should not have been such a new topic.
Considering the length of time that this feature has been around I would consider it a customary feature. Changing to other keywords would probably cause more hickups than good.