This trope is coming up again, yet it still seems to me quite mistaken. Suicide bombers are murderers, but there seems to me little logical reason (as opposed to the emotional gratification of insulting someone evil) to call them cowards.
The obvious point, of course, is that cowardice usually consists of fear of death or injury; the suicide bombers pretty clearly embrace their own deaths. (I set aside suicide bombers who are forced or tricked into being suicide bombers, for whom the analysis may be different.) Now it's true that attacking defenseless people is often seen as cowardice, but that's precisely because most attacks on defenseless people — as opposed to attacks on well-armed people — pose little immediate risk of death or injury for the attacker. Attacking defenseless people with a suicide bomb doesn't share that characteristic. Again, it's evil, but not cowardly.
Some have suggested that the cowardice here is in an unwillingness to confront punishment for their own crimes, much like some might fault a person who commits suicide rather than face a just trial as taking "the coward's way out." But I doubt that the typical suicide bomber is committing suicide to avoid the shame and humiliation of a trial. The crime is just easier to commit if he's willing to die. (A few people may in some situations commit suicide to prevent being taken prisoner and forced to divulge the names of their comrades; but while this may reflect their knowledge of their limited capacity to withstand various kinds of pressure, it generally isn't seen as cowardice. In any event I doubt that it's a big part of why suicide bombers choose that tactic, though it might be help their leaders choose it for them.)
Here's a thought experiment that I think helps put this into perspective: Imagine a just war in which a soldier volunteers for a "suicide mission," in the sense of a mission that is extremely likely to lead to his death. Say, for instance, that we need to knock out the place where the North Koreans keep their nuclear weapons, and the only way to do it is by sending in a force that's nearly certain to get killed (either because they use specific suicide tactics, or because they'll be so overwhelmingly outnumbered in such a tactically adverse situation that there's nearly no chance that they'll get out alive, even if there is a substantial chance that they'll accomplish their objective).
Naturally, our force will try to do what we can to make the target as defenseless as possible, for instance, by using extreme stealth. Though the guards are doubtless armed, our force will try to keep them from using their arms, sneak up behind them and cut their throats, use camouflage, and so on. Then it will destroy the installation, incidentally killing many people who are present; but in the process, our soldiers are sure to get killed.
The members of the force are willing to do that, because of love of country, desire for revenge (if, for instance, their families had been killed by the enemy), or even religious zeal. Some might even use their religious faith as consolation, thinking that their actions are righteous, and that they are trading off a life of emotional pain (again, assume their families had been killed) for heavenly bliss and reunion with their loved ones.
Surely this isn't cowardice: It's embracing danger and likely death, not evading it. To the extent the soldiers do try to evade capture, they're doing it to make the mission successful, not to decrease danger. The fact that they're avoiding trial or killing people who can't defend themselves (because they sneak up behind them) doesn't make them cowards — that's what they need to do to get their task done.
And their sense that their lives won't really be over, but that they'll instead go to a better place, doesn't make them cowards, either. One can, I suppose, say that it takes somewhat less courage to give up your life if you're 100% confident of life eternal than if you think your existence will just end, or if you're not sure. But this doesn't make the decision to give up your life when you're confident of the hereafter cowardly; at most, it makes it a bit less courageous.
Naturally, there's a huge difference between these people and Islamist suicide bombers: Our soldiers' actions are moral and even praiseworthy (because their ends are sound and because they are targeting a legitimate military target, and causing death to civilians only incidentally to that), while the Islamist suicide bombers' actions are immoral. But the difference has to do with morality, not with cowardice.
Again, I understand the emotional appeal of heaping all sorts of scorn on evil people: They're not just evil, but they're cowardly. And they're ugly, idiots, and probably lousy lovers. But it seems to me that this emotional relief comes at the price of logical error.
UPDATE: Chris Lansdown puts it well in the comments: "[T]argeting a group that's less able to defend itself to increase the likelihood of success rather than to decrease the likelihood of personal peril isn't cowardly, it's dastardly (which is worse, though a different sort of malfeasance)."