And in my personal experience, my mother experienced precisely what you don't think ever happens. A sinus infection went untreated for weeks after repeated doctor visits.
It is not that the doctor won't treat a sinus infection, it's that he won't _diagnose_ a sinus infection because the level of care drops when a patient is obviously overweight.
What some docs seem not to have learned is to ask the first question you learn to consider in tech support (and medicine is, in a sense, tech support for the human body) -- "what's changed?".
Unless the patient had a sudden and rapid change of weight that corresponds to the symptoms occurring, even if it is something whose onset was brought on by weight over time (or just made more likely by weight), its probably not the weight itself directly causing the symptoms, and if you don't properly diagnose the cause, you have no idea if losing weight is an appropriate remedy, irrelevant to the symptoms at hand, or something likely to cause changes in the body which exacerbate those symptoms.
Failure to properly diagnose the actual problems makes it harder for the doctor to assist the patient in actually losing weight. "Lose weight" is an outcome, not a course of action, and choosing the most medically appropriate course of action to achieve that outcome is down to coincidence if you decide to skimp on proper diagnosis just because you think that the patient needs to lose weight.
This kind of language is part of the problem, shifting blame onto actual patients.