The doctor being included obscures the point the cartoonist is making, I think. Most of us were raised to view doctors as authority figures, or at least worthy of our high regard and respect.
All the other humans represent people in positions that are likely to be treated like shit. A portion of the general public are known to treat them as less than worthy of respect and civility. Servers, the homeless, strippers, musicians (other than symphony orchestra members), and cashiers - all of them - are seen by the assholes in our midst as legitimate targets for scorn and a cold shoulder, or as verbal "punching bags" whenever such a fuckwad feels like venting. Any one of us on this forum who have worked in retail and in restaurants can attest to that.