Mole,

What exactly are you afraid of the gun doing? You never answered on what you expected the gun to do or why it upset you. If your child had been hurt by a criminal in the store, wouldn't you have been yelling the store didn't do enough to protect you?

The officer had probably been hired to work security for the store. Most cities allow their police to work as security while wearing their uniforms.

You say no maniac was near by, how do you know this? Maybe there was one, but the presence of the cop prevented a kidnapping or worse. That is why security is effective, why concealed and open carry of firearms is effective, It prevents problems from occurring.

The officer was probably walking around the entire store and when he saw people in an area, he went to check them out. Isn't that the job of security?

Please tell us how having a gun is overkilling and unreasonable? Designating a place as a gun free zone is not a good idea. Just ask the kids at Columbine, Virginia Tech, the Cleveland High School.
Would you be willing to do violence to protect your son? If so, how is it different if you are willing to use your fist, the ax in your picture, or the cop carrying a gun?