From what I've read and understand of mass shooting, while they do often occur at gun free zones, they more so seem to target what I would call soft targets.
Well yeah, this is just plain smart. If you goal is to kill people, going after those that are mostly likely to defeat you quickly (unless they are your specific target) means likely not making your goal. However, despite numerous police station shootings and attacks on police officers, armored cars, and the like, much or most of America is a pretty soft target. It is much harder to find hardened targets to shoot up than soft targets here in America. I would hazard a guess that 99% or more of small businesses don't have armed security, >80% of not-for-profits and >80% of major companies don't have adequate armed security or don't have any at all. When they do, it is at specific facilities only and not throughout the corporations.
Glenn noted this earlier, but a LOT of these shootings are not random location shootings. Some are, but a lot are specific to problems that the shooter has with people at the given location. Typical of these are school and workplace shootings. When an employee at company X gets into trouble at work, is picked on by coworkers, or gets fired, he doesn't drive cross town and shoot up company Q. He goes back to where he had the problem at company X. Harris and Klebold and numerous other school shooters who were students shot up their own schools, not somebody else's school.
In short, the targets aren't chosen for being soft per se in most of the cases. Softness seems to have virtually nothing to do with the targeted location relative to the reason for wanting to kill peoplee at the location.