A police officer discharging his or her firearm under any circumstances is in for a major headache anyway (to say the least). Though as I said, he's probably doing it to be a pain and make a point. He likely isn't all that concerned that he's going to cause someone who just shot (at) him issues, he probably feels she deserves the stress. Bruce is just a person, he's not immune to fits of pique. It's mildly assholeish behavior, but doesn't necessarily make him an asshole. I'm not saying it was right (or even reasonable) to grab the gun, but I also don't think it even ranks in the list of 'Batman's big dick moves.'