Terrorists like surprise attacks.
Think about it. You want to surprise someone with an attack. Its no fun hitting them when they expect it, the imact isn't the same.
So you wait until your victim is unsuspecting.
Of course, if your victim goes on alert, you wait until he calms down before hitting him.
IOW, announcing warnings of impending attacks might be a counter-intelligence game to make the terrorists think we know more than we do and get them to lay low until the heat is off. Sort of like walking into a darkened alley and announcing, "don't even try it, I know you're there and I'm ready for you" when you don't know anything of the kind.
Its an idea. Might be true. Probably work for a little while to buy some time.