That is an anti-survival instinct. While you're shooting at the dog, the cops are going to be shooting at you. Your chances in that fight are pretty damn slim.
If a police dog ever comes at you, you put your arms in the air, don't try to run, and don't offer any resistance, and the dog will not bite you. That is how they are trained.
It's not worth the risk. However, as this case demonstrates, it may well be better in some towns to fire at the cops first, then the dog.
Don't worry, the lawyers taking this case are going to come down hard on the "trained dog" angle.