Reason number 3 is simple and financial. Huge money involved in sporting events live and in person, not to mention the monetary endowments Universities receive for the teams. Scare people away from that (and pro games) and some big dollar folks are going to come up waaay short in the till.
True. But I think they underestimate the American people. We've had plenty of movies about terrorists blowing up stadiums. We've had plenty of warnings about subways, railways, and malls. But people still use these things.
Since 9/11 there have been increased guards in Penn station, sometimes sniffer dogs, sometimes troops in fatigues. There are constant warnings from the loudspeakers not to leave a bag unattended. But people still use Penn Station.
People can adjust to these threats, once they are informed. An informed people is better able to defend itself, and to pull the lever for the right sort of politicians to help the defense.