We have it because of a few isolated, but nonetheless shocking and highly publicized, events which caused a great deal of irrational fear in voters.
Laws should NOT be made in that kind of atmosphere, but as we all know, they are and will continue to be. The PATRIOT Act, CFR, AW ban and a host of others are testimony to this quirk in our system.
It is also the reason why the Left always seems to call everything a "crisis".