While many public shootings take place in gun-free zones, not all do. The Tuscon and Aurora shootings did not. Furthermore, I don't recall any of the surviving gunmen having explicitly said they deliberately chose gun-free zones.
You are correct that the Tucson shooting did not occur in a gun free zone. Most classify this as a mass shooting. In reality, it was an assassination attempt. There was one CCW'er carrying in the Walgreens next door to the Safeway. His name is Joe Zamudio and I've met him (he shoots regularly at the range I shoot at). If he had been on scene 10 seconds earlier, it's likely he would have saved lives. But by the time he was on scene, the brave heroes who were there had already disarmed the attacker.
However, the Aurora, CO shooting was different. The municipality of Aurora had a strict law on carrying guns (basically impossible). The thing about it is it's unenforced because of Colorado's preemption law. The movie theater, however, had a "no gun" policy in affect. Even though in CO, no gun signs do not have force of law (you can be asked to leave if your gun is spotted) many people don't realize this. Technically, guns were not allowed in that theater. However, it wouldn't have been illegal for someone to be carrying. If we're looking to "no gun" areas as being a possible motive, the shooter might not have known the law well enough to know that it was legal to carry there.
I basically agree with you that "no guns" is not always a motive. But it's interesting how the majority (there's always an exception) of mass shootings occur in places where guns are not allowed. Ft. Hood (even though it's military, very few, if any Military bases allow anyone to carry except for MPs), Columbine, VT, Norway (the killer chose the location specifically because he knew LEO response would be slow, and no one had a means to protect themselves). There's more than that, and a common thread is that they occur in areas that don't allow guns. Whether that's part of the motive is up for debate. It could be because mass murder attempts in areas that allow concealed carry are stopped before they get bad. Maybe not, I don't know.