[Rolo]

Sure, it would be nice to know the time and place, when the next terrorist attack will happende, but this is not the weather forecast - terrorist might be brainwashed by religion, but just like any other military operation, then the terrain decides what kind of attack is to be used, and terrorists can change/modify their plans.

If the goverment told the public, that the attack might come from the sky, then everyone would be looking at the sky, and not notice that the terrorist changed his/her plan to another target.

Also, by not giving too much information, then the terrorists will not know how much the goverment knows already, which will help the goverment when they strike against the terrorists.