Here is where your argument fails in real life, WinThePennant:
1. First, let's assume the bad guy is capable of intelligent reasoning. If he/she is evaluating targets, there is simply no reason, whatsoever, for them to attack the person or place where there is a visibly armed citizen present. Since the assumption is that they are logically and thoughtfully evaluating targets, it would be an equal assumption that they have a logical and thoughtful desire to complete a crime without getting caught, without getting shot, and without turning a robbery into an attempted murder.
Given the parameters of intelligent reasoning, it only takes an ounce of intelligence for the criminal to figure out that there is only about .5% of potential targets that are visibly armed - so why not move on to the any part of the other 99.5% of targets that aren't? As evidence, look at the Kennesaw GA Waffle Robbery that was diverted due to the presence of armed citizens.
2. The Columbine, Ft. Hood, Sen. Gifford, etc. shooters can hardly be considered to be in a logical and thoughtful frame of mind while they are blazing away randomly trying to kill as many people as they can, or focused on one particular target such as Gifford. History has proven that they aren't going to evaluate every person in a group to see if they can detect a gun or a person open carrying. They go in and shoot as many people as quickly as possible, or their particular target as quickly as possible. Even in storm in and shoot robberies this proves to be true. Look at the open carrier with the SA revolver that killed the convenience store robber in Richmond, VA.
The simple truth is that your theory does not and has not proven itself to ever occur in real life because you are attempting to combine two criminal mentalities that have, so far, never combined in reality. That is the criminal who wants to shoot a group of people and carefully evaluates who may or may not be carrying a gun to shoot first. That criminal simply has not existed, yet, in reality.