So what was supposed to be done? Allow aircraft to continue to land and arriving passengers to crowd the terminal whilst the police and airport/airline authorities were endeavouring to determine whether the note was a hoax?
It's easy with hindsight to criticise but I have no doubt the criticism would have been of a far different order had the authorities not taken such action and a device and possibly secondary ones around the airport had detonated killing and maiming hundreds of people.
An airport doesn't have to be a major one to be a terrorist target.