The perp may be an idiot, but so is the store clerk.
First, he had better have been 110% certain that the perp had a BB gun, or he put himself in a Mexican standoff, which is no better than having no gun at all and just daring the perp to shoot you in the belief that they don’t have the nerve.
Second, he comes very close to having his gun taken away from him by the perp, by very stupidly trying to shove it in the perp’s face.