Well, 6 shots from a 9mm or other small-caliber handgun can come pretty quick. You can probably squeeze off 6 rounds before someone has a chance to comply. Almost certainly before you, the shooter, can realize that they are trying to comply.
But, that's kind of beside the point. An officer doesn't use a handgun (or any gun, for that matter) as a means of "compliance". They don't think or say, "Let me wing this guy and see if he complies by getting on the ground." That's what tazers and pepper spray are for. You pull the trigger to kill. That's it.
Now, if you miss your intended target (center mass) and only wing the guy, and realize that he's surrendering, then you no longer have a right to shoot. But, that's a very hard decision to make when you're shooting a gun - to figure out that the situation has suddenly changed from, "my life is in danger and I need to kill this other guy" to "oh, he's not trying to kill me anymore and is surrendering". Tough, but that's why they're paid the big bucks (wait - seems like a conversation we've had before).
To be clear, I'm not saying that it's "OK" when a cop keeps shooting in the first few seconds that a person is trying to surrender. It's definitely not. But, I can understand how his brain simply hasn't switched gears yet. I've been in a fight, both on the street and in martial arts matches, where the other guy "gives up" and I keep going for a second or two, because I'm pumped up on adrenaline and am not thinking rationally/calmly/etc. Doesn't make it OK, but I know the feeling.