My question is essentially, what are the philosophical reasons behind the way in which societies as a whole react to terrorism. Taking the recent Manchester attacks as an example, 22 people were killed in a bomb explosion. On average about 1400 people die a day in the UK. Now what is it about terrorism that attracts the outcry/policy changes and general disproportionate impact upon society relative to the absolute damage caused?
Terrorism is classified as war and thus understood in the context of war ethics. Traditionally, the ethics is divided into two considerations: when to go to war or who has the moral right to start a war (called jus ad bellum), and what should be morally permissible ways of acting during the war (called jus in bello).
Under jus in bello, citizens are viewed as non-combatants, and thus as being innocent. Thus argued is that terrorism whose nature is the attack of citizens is morally impermissible. Viewed in this light, terror acts attract public outcry for the acts' moral impressibility.
Now the story does not end here. Many terrorists regard themselves as freedom-fighters, and regard their cause as being just. This means that they can appeal to jus ad bellum. That is, in a way they believe that they have the right to start a war. They then argue that in a democracy, all the citizens are morally, collectively culpable when their leaders engage in unjust wars. After all, it is the citizens who chose their leaders.
There are articles that concede this point, yet try to show why attacking democratic citizens is nonetheless morally impermissible (.e.g, terrorism makes people live in constant fear, which is despotic. Meeting injustice with another injustice thus cannot be justified.)
First and foremost, this is a tragedy for all those directly affected as well as so many people in the wider society. I hope that the focus in the response is not offensive or callous towards this or any other event like this.
I would argue that the OP reference to a "disproportionate impact" is not appropriately contextualizing the event. The "absolute damage" the OP describes is quantified by the human casualties of the attack. However, the purpose and damage of a terrorist attack is wider than the tangible impact. The damage extends to the creation of fear and disruption. Demonstrated when attacks do not include any military or political target, the intent is to strip any sense of security which is foundational to society and to chip away on the morale of a population. This demoralization is a historically significant mechanism of attacking an enemy in many forms of competition; psychological warfare, ideological conflicts, politics, etc.
It is important to counteract these effects and reinforce the collective unity. To do this, we share in the pain and loss. We demonstrate strength through support. If we are undeterred to live our lives together and not dissolve into hate, it is a triumph over terror.
I assume that is because, in our view, the victims' deaths were unfair and unjustifiable while we do not care about the thousands deaths that occur naturally a day because it's the natural course of events. Then again, media make money out of news like these. News spread and we start thinking "what if we were in that situation?". The fear of death/the sorrow for our relatives' death makes us feel horrified about what happened.