When a police dispatcher receives a "man with a gun" call, I'd guess the caller normally does NOT indicate if the gun guy appeared to be a nut case/wacko or if the gun appeared to be a handgun, rifle, shotgun or a machine gun or even a bazooka. I believe most of the liberal public doesn't give a shit what type it is, just that it's a damn gun, capable of doing some serious harm and some guy is out on the street with it. I believe the cops really have to check out what's going on. If they did NOT investigate, and the carrier shot someone soon after the call, there would be more than a few complaints ("well, I called the cops and they did nothing about my citizen complaint/warning").
For the cops, it's hell if they do and hell if they don't. I believe they'd rather go with the safest method and just routinely check the guy out. Also, just because he (the open carry guy) cites some laws indicating that open carry is legal in that area, that does NOT mean that he is certainly a law abiding citizen. You can bet that a lot of guys who are currently incarcerated are very familiar with many many laws and can easily cite them but that doesn't suddenly make them "good guys".
A cop/friend of mine told me many years ago that a cop frequently may have to make some quick/split second decisions that may take the courts months or even years to unravel and figure out.