Um... because that's what they said they said it was for? "According to the ministry, the app would provide users with alerts and information within 15 minutes of a terrorist attack being confirmed." It's a simple case of the current guidelines determining when it should be used.
They can certainly change the change the rules for when they use it and, instead, send announcements when something is suspected to be related to terrorism or even simply when something tragic happens.
Of course the next step for the next event would be to question a) who said it might be terrorism or b) was the event a big enough tragedy.