An explanation of the KNZ in STII could be that it was a simulation along the lines of expect the unexpected, so it doesn't have to align with "reality".
One must look on the reason for creation to guess the function of an NZ.
The Romulan one was created after a war, so it serves more like a buffer, or perhaps the Iron Curtain is a better example.
While the Klingon one was more an Organian no-fight zone. So theoretically a Klingon fleet could cross the Zone and conquer Federation worlds. And there were concrete rules as to to whom a planet would belong.
But it is true that the NZs don't seem to work as Federation/Romulan and Federation/Klingon ships see each other face to face too often for my tastes.