Plus, middle and upper middle class people are more likely to pay any balance that's left if their insurance doesn't pay the whole amount (not unusual for private insurance). Yes, they have more money, but more importantly they generally feel more of a sense of obligation to pay their bills.
It is generally true that the vast majority of the poor on medicaid feel that they shouldn't have to pay anything at all for medical care. It's part of the entitilement mentality that seems to arise with years of dependence on government support.
I worked in a hospital based urgent care clinic associated with the ER prior to med school. The vast majority of the population was medicaid patients. Prior to that clinic opening (or during the hours that the clinic was closed), the patients had come to the ER for all kinds of inapproprate things...like head lice. They also tended to get really upset if they had to wait, yelling at the triage nurse. She said one day, someone yelled at her for having to wait (he'd been there less than an hour) and she apologized, saying they were dealing with an emergency (a code was going on, and in this rural hospital, that basically took the full attention of all ER staff). The man became really irate, saying "Every time I come here you are working on some kind of emergency! (go figure)
Anyway, medicaid patients don't generally think they should have to pay anything for medical care out of their own pocket. They come to the ER for lice, because if the have a prescripion, medicaid will pay for OTC lice treatments (no private insurance does that!!) Same is true of tylenol and ibuprofen. And, while yes, some people probably can't afford it, many are yacking on their cell phones, driving new cars, discussing the latest cable TV program. And many of these people simply don't think they should have to pay any balance on a medical bill not covered by insurance. (That's why you find so many people in the medical field get very cynical very fast)