It's an interesting concept, but makes that assumption that first, all the clerks are capable of throwing a ball, and second, that they can actually hit something they throw at.

Without wanting to be sexist, many girls who haven't been involved in sports tend to throw a ball as if they have one foot stuck in a bucket and could only be certain of the ball eventually hitting the ground.

IMHO, a better system would have a fixed sprayer at the door activated by a two stage trigger that first requires the clerk to press switch and then sprays when the door is opened.