The answer is because they know Americans are ARMED to the teeth.
Do you REALLY think that your .45ACP or 9mm inside your waistband will scare off terrorists who are willing to blow themselves up in the process?
Sheesh.
Also, since Obama took power the malls have been pretty much empty, so the joke would be on them.
Don't put to much stock in that preventing something like this in the USA.
Lets say on an average day at the average mall there are 2000 people at the mall. Of those 2% have CCWs because they ignore the gun-free zone signs.
I am sure they could have some positive effect perhaps even taking out a few of the scumbags, but in the end the death toll would still be very high.
The scumbags want to die, they have no fear of death.
The only way to prevent this happening here is to ensure that they never, never enter the US in the first place. But thanks to our wonderful congress writing insane immigration policy the gates are wide open and its only a matter of time.