The medical buildings that impose that charge all seem to be located in the "richer" parts of the county. Most of the buildings elsewhere have free parking. I should add that this isn't only medical buildings, it's also the kind of buildings that have lawyers' offices, financial analysts, etc.
Of course, anyone who can't afford the parking or strongly objects to it always has the option to find a doctor or lawyer in another location. But what if they have a serious medical condition and want a doctor who has an excellent reputation and is located in one of those buildings that charge for parking? Most people wouldn't risk their lives by hunting for a free parking lot. They just pay it.
Of course, in those same upscale communities, you also have to pay for parking at many of the restaurants, theaters, and sports venues. (Don't get me started on the sports venues - they're in a class of their own!) But at least those things are pretty much optional. Going to the doctor, and sometimes to a lawyer, isn't always optional.