If a plane goes down due to a terrorist attack those unfortunate people end up dying. If you just calculate the cost of saving those people by implementing tough security measures you can argue that it's not cost effective.
BUT you can't just figure the lives of the people on the plane into your calculations. You have to figure the impact to the airline industry if people become afraid of flying and what will happen to the US economy when another successful attack comes.
All three (lives on plane, airline industry and economy) are worth having tough security measures in place.
So if they stop all of these security measures and people die as a result, you aren't going to blame the TSA or the airlines..right?
Edit: I found a paper from 2007 that claims there were 129 extra driving fatalities in the last quarter of 2002 due to harsher airport security. http://aem.cornell.edu/faculty_sites/gb78/wp/JLE_6301.pdf (page 27)
Or that backscatter X-ray machines odds of giving you cancer are roughly equal to those of dying in a terrorist attack: http://cosmiclog.msnbc.msn.com/_news/2010/11/16/5477568-are-...