Do you have any knowledge on the history of domesticated dogs? The whole reason pits and rotts exist is because they were selectively bred for size/power/fighting prowess. They're a creation of our historical desires for them.
I am NOT saying that people shouldn't be able to own them, or that they are inherently bad dogs, I don't think Preslove is either. I've actually met easily 10x the nice playful energetic happy pits than pissed off neglected/abused/angry ones.
What shouldn't be brushed under the rug though is that Pits and Rotts are just hugely more powerful/dangerous IN THE CASE that they happen to go on attack mode. The chances of being attacked by any particular breed of dog is an almost meaningless statistic, hell I've probably been nipped/snapped at more by tiny dogs by far than large ones. The point is that when a dog of the caliber of a Pit/Rott is on the attack, you've got a dramatically raised chance of being seriously maimed if not killed. Closely following those two are two other exceptionally agile breeds that fight very effectively : Dobermans and German Shepherds, pound for pound about as tough as they come.
All of this is common sense anyway.
I wasn't at the scene described in the OP, and neither were any of us. I happen to be of the opinion that it sounds a little shady, as GR typically don't present much of a threat, even if irritated, to an adult male. If it were a Pit or Rott that were chasing the cop, I'd have absolutely zero doubts about the need for immediate deadly force. A GR though? I have my doubts that it was really necessary. It could just be a freak thing though, I mean how many fatalities or serious maimings were caused in the past decade by Pits? Alrite, now how many by GRs? It's a pretty disparate situation.