The TSA control what can fly and what can't. Unfortunately, they're a tad retarded with regards to what can actually kill.
The idea is that making average fliers more miserable will somehow protect them from terrorist attacks. Such restrictions include not being able to read books, having to have special bags for laptops, not being allowed more than 3.4 oz of any fluid (such as shampoo, juice, hair gel, milk), not having sippy cups or bottles with beverages for children, not being able to bring a wrapped-present to your aunt in England, not carrying snowglobes, and increased discrimination if you have darker skin.
You can, however, bring various types of weapons. Just ask The TSA themselves.