Because deep inside they too despise the United States and Israel.
America (& Israel) are at war at this moment.
If they're at war, it means there is an enemy. If there's an enemy, he can strike anywhere, any moment. You, American cowards, think the war you're waging always happen far away from home. Very heroic, indeed. It's sad for the kid, who is absolutely innocent but if the other victims were Obama or Romney voters (which isn't necessarily the case, they may have been abstainers like 60% of the population, and in that case it would be very sad too), they've just been war casualties, period. I'm not celebrating but that's the way it is.
The only problem is, the two terrorists were no Afghans, Iraqis, Syrians or Lybians but manipulated Chechnyans. While America has always been one of the biggest support to the Chechnyans Wahhabit terrorists. Which means it was a false flag, no doubt. Of course, I'm gonna be considered a CT but I'm a Euro and I remember that here in the 70's/80's (the Years of Lead) most terrorist attacks were committed by manipulated extreme-left or neofascist terrorist groups (we now have evidence for this: the famous strategia della tensione and Operation Gladyo) like the Bologna Station Bombing and the Munich Oktoberfest Bombing, both in 1980.
From this we may easily conclude that the Islamist groups, just replaced the extreme-left groups of the 70's/80's (Brigate Rosse, Action directe, RAF, CCC, etc.)