We don't. Nearly anyone who is armed on just about any school grounds (K-12 and university) in the US is committing major state and federal crimes. So we're not "letting". It's against the law. And curiously, that fact isn't stopping intent crazy people from doing it, even through they "know it's wrong and really bad thing". Odd, isn't it? Almost insane. One might even say it's a crime and there autta be a la... oh wait, there is.