With that thinking the police can do what they want when they want is extremely dangerous thinking.
Of course it is but cops do it and, in many (most?) cases, get away with it. Even in this situation, the vet, operating within his constitutional rights, was still arrested and will have to defend himself. What is likely to happen to the cop? If not for the video tape, the answer is probably nothing: the judicial system will protect its own.
A lot of cops bad behavior happens in poor, minority communities; out of the glare of the press or dash cams; dash cams that have intentionally turned off by the cops. And because many of these communities also have high crime rates, it is easier for cops to validate their bad behavior. Cops have been bad actors for a long time in these communities and because of this, many people who live there have a totally different view of cops than do people in non poor, non minority communities.
My point with the comment to "put yourself in the position of the cop" was to suggest that the law allowing open carry is in conflict with human nature; especially given the heightened awareness surrounding gun related deaths.