That doesn't follow. The TSA may be deterring terrorists for airlines, or they may not.
But the fact that they don't protect other things, like malls, doesn't change that possibility. You are right terrorists can target softer targets, and have recently done just that. And it may be because because of heightened security at other places that they are starting to choose softer targets. If you want to kill a lot of people and get a lot of attention, go where lots of people are that will have a big media reaction and where the security sucks.
But the fact that they don't protect other things, like malls, doesn't change that possibility. You are right terrorists can target softer targets, and have recently done just that. And it may be because because of heightened security at other places that they are starting to choose softer targets. If you want to kill a lot of people and get a lot of attention, go where lots of people are that will have a big media reaction and where the security sucks.