Dave Roberts points out
that, if you assume that "harm is harm; death is death," then our worry about terrorism is grossly disproportional given how few people are actually killed by terrorism. (Indeed, if this wasn't true, it wouldn't be terrorism -- the whole point of terrorism is to generate huge fear through a few dramatic killings.) The real big killers are things like poor eating habits and smoking.
But it's not enough to just consider the comparative harms -- you also have to take account of the benefits. Smoking and eating junk food have big benefits to smokers and eaters. On the other hand, terrorism has no benefits for its victims. But getting worked up about terrorism does have big benefits for potential victims -- it generates a sense of solidarity, and flatters our machismo when we take "tough" stands against it.
Even with the benefits side of the equation factored in, the numbers still don't seem to add up. The fact is, people don't
believe that "harm is harm; death is death." Roberts is right that one of the key differences between terrorism and poor diet is that terrorism is something done to us by someone else, while the harms of poor diet are self-inflicted. To some extent, a fear differential here is rational. fear is a response to uncertainty and lack of control. Things other people do to us are more unpredictable and uncontrollable than those we do to ourselves.
Roberts offers a sociobiological explanation for why we fear external threats more than self-inflicted ones. On the savanna, it was evolutionarily adaptive for primitive tribes to panic over external threats, but that adaptation is out of place in our complex modern world. But is that really true? Whatever the dangers of intertribal warfare (which, so far as we can tell, varied widely in time and space), there were certainly plenty of dangers that could be self-inflicted. Laziness could cause malnutrition, inept hunting could get one trampled by an elephant, inept gathering could result in eating poison mushrooms or drinking impure water, camping in the wrong spot could get you washed away by a flash flood or frozen by a blizzard.
I don't think a single direct explanation, of the type Roberts proposes, is sufficient here. Cultural theory tells us that we fear risks that are immoral. Moral outrage arises from violation of accepted social structures (the generalized building blocks of which are doubtless evolved, but whose specific applications are contingent and cultural). According to Alan Fiske, there are four such building blocks -- ingroup/outgroup boundedness, ranking, equality, and freedom. Thus I would suggest there are at least four key triggers of fear: infiltration/profanity, insubordination, unfairness, and tyranny. Terrorism is able, in our culture, to set off all four triggers (albeit different ones to different extents for different people -- a crucial caveat in all such discussions). Poor diet can perhaps trigger one, but even that is mitigated by our modern liberal (in the sense of the broad historical tradition, not the contemporary political agenda) culture.
Terrorism is infiltration -- unbeknownst to us, outsiders are able to slip into our society, exploiting our institutions in order to destroy them. Terrorism is insubordination -- there's an established pecking order in the world, in which the USA is the alpha male, and organized states with uniformed armies stand above non-state actors. But terrorism is precisely an attempt to overturn this order. Terrorism is thus also unfairness -- terrorists refuse to play by the post-Westphalia rules of political struggle, inserting both themselves and civilians into a form of conflict that is supposed to be reserved for uniformed armies. Finally, the particular form of terrorism we face now is tyranny -- the popularity of the term "Islamofascism" shows how we concieve the terrorists' goal as the establishment of a theocracy repressing legitimate freedoms. (Anarchist terrorism would lack this trigger, but it would get a double hit on the "insubordination" trigger, since it aims at the overthrow of all rankings.)
Poor diet is not tyranny -- indeed, it's defended as an expression of personal freedom to eat what we please. Poor diet is not unfair -- indeed, criticism of poor diet is (inaccurately) derided as elitist and proposing solutions beyond the budgets of normal people. Poor diet is not insubordination -- indeed, eating Big Macs shows you're a regular joe, whereas eating tofu is either elitism or the converse crime of effeminacy. One can potentially see poor diet as a form of infiltration/profanity, in a way similar to how sodomy is seen by some as a crime against (an infiltration or profaning of) one's own body. However, that framing has little traction in a modern liberal culture (even sodomy is more feared due to being seen as insubordination). We insist that one's body is one's property, to be done with as we like. What's more, unhealthy foods are accepted parts of our society, so it's difficult for the average American to conceptualize them as dangerous outsiders.