From a legal perspective, it is relatively easy to prove to a jury that the person holding the weapon murdered someone; it is much more difficult to prove to a jury that there are solid connections between the murderer, his ideology, and say, some guy who donates money to charity and sends funny notes from obscure parts of the world about 'big weddings.'
From the standpoint of trying to defend the country, we must remember that our military is capable of winning any conflict- but it can win long conflicts only with the support of the population- in other words, public opinion.
In order to win a war over the long term, it is important to keep the not-so-perceptive people from getting hysterical and flighty, making the markets and economy unstable, or making their popularly-elected congressmen think more of reelection chances than winning a war. Leveling shrill and thoughtless demands that we pull out from the conflict and hide with our heads in the sand and our tails between our legs will never result in victory.
There are people who go bonkers if they three-inch high fonts in their headlines screaming TERRORIST LIVING NEXT DOOR, and they overeact, make runs on the banks, dump stocks, call in debts prematurely, overdose their kids on Cipro, hide in their basements, fail to show up for work, shoot anyone with a turban on their head, etc. Hysterical people aren't any good at all for the economy or defense; a bad economy impairs our nation's ability to persevere, and panicky people with hairtrigger fingers are more likely to get our second ammendment rights and other rights completely trashed than they are to save anyone from a terrorist attack.
Terrorists don't expect their deeds to destroy the US directly, nor even seriously hurt us. They seek to destabilize society by making people panic, become disruptive, and act impulsively on their fears. They seek to make the population of a target nation undermine that country's ability to defend itself.
That's not to say people shouldn't be warned about terrorist activity- but those able to understand and adjust to the threat can already see what is going on without officials drawing pictures of the obvious. Those who are so slow they need it spelled out for them in an obvious fashion aren't going ot be able to do anything with the info anyway- except maybe pee in their pants.
I have believed all along that the reasons you stated also apply to our government officials all the way up to President Bush. Given the slightest hint that Islamists have declared war, and every trigger-happy whacko would come out of the woodwork.