I think that they target civilians because that is the best way to instill fear and constant worry within a population. When people are fearful and agitated they tend to act differently, they might ask more of their government, or change their minds believing the government cannot keep them safe. Also, sometimes the goal of these terrorists is to have the government step up their security, enforcement, and regulation (many times this means restricting freedoms of average citizens), and when these freedoms are further restricted people start criticizing the government calling them tyrannical, oppression etc... (just look at the response the Patriot Act got). I think the best way for these terrorists, or lone wolf madman, is to achieve their goals is to cause panic within. Attacking military, police, etc... would be a manly thing to do, but it would also mean you would get you ass lit up by well armed well trained soldiers, officers etc... so the safest way is to attack civilians. Also if you want media attention attacking civilians is the way to go (unfortunately), the more gruesome the act the more attention one will get. I'm pretty sure that some international terrorist organizations attack the U.S or soft targets within their borders in efforts to get the U.S involved, and to make their own governments step up police powers. Once this happens, people feel that the their government is tyrannical, they see the U.S as a foreign invader, and all of a sudden ppl tend to become more polarized and more sympathetic toward the terrorist cause (this is foreign countries). Here we have a pretty strong social framework and I don't think our system will ever collapse because a terrorist attack. However, these attacks will prompt gov response that will be tempted to do something, and when government does something usually it's in the form of some kind of law or regulation. Perfect example of this is guns. One madman goes on a shooting spree, everybody lashes out for the government to keep them safe, gov introduces a new "assault weapons" ban. Fear drives ppl to do irrational things, gov to step up enforcement and enforce more stringent rules, and some valuable freedoms can be on the receiving end of all this panic and commotion. That is my best guess as to why they choose civilians. Although, I am by no means an expert in this area.
