While I agree with you, there is interesting data about effectiveness of these things coming from Israel:
Back in 2000, Israel had a series of suicide bombing attacks, which resulted (among other things) in a law requiring every mall, restaurant, cafe, theatre, office buildings and many other businesses to have a full time security guard who will check every person coming in with a metal detector.
There were a lot of cries about how useless and what a waste of money that was (as, of course, the cost was shifted to the customers). However, it turned out to be a deterrent in a strange way:
There were several cases of suicide bombing later that year, in which the bomber saw the guard, and decided to go to an unguarded place (most establishments observed this law, but not all). In one case, there was no visible unguarded place, so the bomber blew themselves up in the middle of a busy crosswalk.
I think the bottom line is that these measures did not change the overall outcome, which was just as tragic, but they did manage to shift the locations of the events.
It's like IT spending for companies that have nothing to do with technology (think about a paper supplier): You have to do it because everyone else is doing it, and if you're the only one NOT doing it, bad things will happen to you.
But once everyone is doing it, everyone is no better off (earningwise) than where they were before.
You have to keep running to stay in the same place.
Just as a note, the vast majority of schools in the US still don't have metal detectors despite incidents like Columbine. Talk about that sort of security theater, and imposing the costs on the businesses as the government wouldn't pay for it themselves, seems overblown to me. All that would happen is that the targets would shift to other, less protected, sites: malls, shops, restaurants. It would be a neverending security escalation, and an enormous financial drain directly on the businesses (not on the fed or the states) that they would likely lobby, effectively, against.
There's a cinema in Valley Stream, NY with metal detectors after a number of shootings in the 90s. I went there exactly once, it's off-putting enough to make me want to go elsewhere.
