I dunno, these fighters train a lot - they aren't just blindly fighting in a state of amnesia and adrenaline. Many times in the past these "unnecessary shots" have been confirmed to be completely intentional... for example Dan Hendo's famous H-Bomb against the KO'd Michael Bisping...

Also, I sometimes wonder it these sportsmen use that tactic to get the dub, and the ref goes along with it for fear of backlash.

The ref doesn't have to stop the fight when a fighter declares himself victorious though I do see what you're saying...

I think the best case for this sportsmanship involves the winning fighter hovering above his downed opponent ready to rain down strikes, and the ref will go "oh shit no" and end it first... or the ref will say nothing, the fighter pops a few punches and then the ref stops it.

The stupid way to go about this is Anderson Silva declaring himself victorious after dropping Bisping, running off and climbing on the cage and cheering himself the winner while the fight is still going on and the ref is shouting "hey, come back - fight on!"... then he proceeded to lose to the judges scorecards.

It can go both ways if you try game the ref, if you just down them and walk off trying to look cool chances are the guy might recover and you could miss your chance to finish.