It may, indeed, be more difficult than tracking what everybody does and incarcerating those that are suspected of plotting acts of violence. I think the payoffs may be significantly better, though.
I hate to compare these kinds of cultural issues on the internet, but I have lived in many places in the world. Culturally they are very different. How we act in our countries, states, cities and even houses is governed by our culture. Some cultures value peace greatly. Some prefer justice to peace.
When you look at the people around you, you may think, "There is nothing I can do. There are evil people and I can't change that. I can only fight them so that good will win." In my experience, this is not true. Many people have problems and it is only society and culture that can save them from those problems.
How do we extend a culture of peace and create a global society where we help people in need? That's what I would rather spend my money on. I'm willing to suffer from the attacks of those that need help in the mean time.