They do what they do, because they know we're too weak to resist striking back.
Our knee-jerk, bomb-dropping reflex is our weakness. They are trying to exploit it, because retaliation bombings are how they recruit more terrorists to their side. And please note that when I talk about their “side,” I'm not talking about Islam, or even Islamic terrorism. Their “side” is what I'm going to henceforth call Team Violence ... The bully doesn't fight because he wants to win; he fights because he wants a world in which everything is resolved by fighting (note: The bully himself doesn't realize this). It doesn't matter if he loses — the moment you chose to fight, his side already won, and the world becomes more like the world he wants to live in ...
In other words, “We can't beat them, unless we become more like them.” It's like a doctor telling you he's going to get rid of your tumor by growing a bigger, meaner tumor next to it. Even if it works, Team Cancer wins, and you just fell for a scam that has been tripping up humanity for 200,000 years or so.