The bag check isn't about weapons. It about trying to reduce any number of items that Disney doesn't want in their parks like coolers, glass bottles, bags of marbles, cell phone jammers, and in AK straws. This is why the guy's duct tape was confiscated. It is about protecting the park not the people. I am sure they keep an eye out for weapons as part of the check but that isn't really the main reason they search a 6 year old girl's backpack.
You can easily carry concealed into Disney World and never be detected. I hate to burst your bubble, but many do...