You're only making such a statement because you know of the events that happened.
The nation was in PLENTY of danger in real time, as it was happening, as evident by the attack on the WTC, PENTAGON, and by the downing of the other plane! No one is going to know exactly what dangerous events will lie ahead, so, to suggest otherwise is pretty ridiculous. Maybe you can't recall what happened during that time.
The NATION wasn't in danger. People were. In certain locations.
I'm making the statements because the event was SPECTACULAR. In this country, and probably the world, only that class of event is worth obsessing over. Three thousand or so people died in those despicable acts. Since then, over a hundred thousand have died as a result of DUI. See any alerts? Why not? The nation is really and truly in danger. Don't you care enough to demand an alert for the thirty people who will die THIS VERY DAY from drivers like that drunk over there?
I knew the NATION wasn't in danger from the time the second plane hit and we knew it was deliberate. So did you. Did you (unless you lived in the target area) beat feet for the basement and hide? I suspect not. I didn't. I left the TV on and checked it occasionally because it was a major and horrible event. But I didn't worry about a passenger plane suddenly plowing into my happy home, or the town hall. I certainly hoped that whoever was doing it had run out of hijacked planes.
To be honest, if I had been in the upper levels of the government, responsible for what might happen, I probably would have it the red siren button, also. Of course, the sirens, alerts or messages wouldn't have had a single effect on anything except make people nervous.
A nuclear device on a city, rockets on radar coming over the poles, the appearance of an asteroid coming this way - ok, that's time to push the button.