The knee of a deer is tiny compared to that of a person. And the knee isn't the only viable target with threat neutralization targeting. It could be the shoulder, hand, foot, lower abdomen. Even the buttocks. Also, with my caliber limitation idea, Zimmerman would have never been approved for carrying a deer rifle.
But your comparing hunting deer to a neighborhood watch is very telling about your mentality.
I'm not arguing the gun is the superior tool if you are only talking about non-lethal interdiction. But that it gives you the option of lethal force that you don't have with pepper spray or a taser.