Well, why should they curb it? Some citizens want it curbed, but that's not always a good reason to curb something. I say look at it from a political perspective. What does a congressman have to lose politically by introducing such a bill? Here's a hypothetical: A congressman introduces a bill, the bill gets passed, and a month later extremists successfully detonate a bomb in a populated area killing several people. It's now election time. Guess what the opponent's platform would most likely include. Probably something along the lines of: "I won't vote on laws that will endanger American lives." Game over for the congressman.

This is just one possibility why we haven't seen any significant movement towards legislation. It sounds oversimplified, but it can make or break an election.