I think the series (and I assume the comic) took their cue from the Romeroverse, where everyone comes back, regardless of cause of death. Those bitten suffer a severe infection and high fever that causes death, but they would have come back anyway, even if they were just hit by a truck.
One thing I notice they continue to play with is the Walkers in a greater state of decomposition as time progresses. If that's true, shouldn't the ZA peter out eventually? Between the dwindling food supply and loss of mobility as the body decays, it seems to me within a couple of years the pendulum would swing the other way, and the Walkers become just an annoyance.'
One of these days someone will create an epilogue where is bullet to the brain at the time of death is just another accepted part of the procedure.