People tend to infer the motives -- and also the disposition -- of someone who performs an action based on the effects of his actions, and not on external or situational factors.
This is relevant to terrorism in the context of the assertion that terrorism is typically not very successful at helping terrorists attain their goals, because victims tend to assume that the terrorists' goal is to hurt them, rather than effecting some political change.
For example, many people probably believe that the 9/11 attacks were carried out because Al-Qaeda wants to destroy the Americal way of life. But the way I understand it, bin Laden's feelings toward America go back to the early 1990s, when Saudi Arabia allowed Western military forces to be stationed in Saudi Arabia, the home of Islam's two holiest cities, Mecca and Medina. The Schneier post lists four other motivations behind bin Laden's actions. Bin Laden doesn't necessarily want to kill Americans for the sake of killing Americans, but rather to change America's Middle Eastern policy. But many people (understandably) have trouble seeing further than Ground Zero, the Pentagon, and a field in Pennsylvania.
This brings me to a very interesting book I recently read: Religious Literacy by Stephen Prothero. The book details how little the typical American knows about Christianity, let alone the world's other major religions. I learned about this book when the author was interviewed on Comedy Central's The Daily Show. Prothero told an anecdote about a government official (someone influential in U.S. foreign policy) who was unable to correctly answer the question "Is Al-Qaeda a Shi'a or Sunni organization?" Prothero's book makes a pretty convincing argument that university and/or high school curriculum programs should include mandatory courses in basic religious literacy, and that understanding religion helps us to be better citizens, better able to make decisions. If you disagree with that thesis, ask yourself a few questions. Do you know what the terms Sunni and Shi'a mean? Do you know why Mecca and Medina are holy to Muslims? Can you name the world's five major religions? To what story was George W. Bush referring when he mentioned the Jericho Road in his inaugural address?