There's a pretty staggering breadth of admissions requirements for the various accredited medical schools in the US, and plenty of offshore schools that effectively will take anyone — basically MD mills. The Dominican Republic, in particular, is well-known for this.
(Yes, many of the offshore medical schools are LCME-accredited and legit, and have trained some fantastic physicians, but broadly speaking, they're taking people who couldn't get into an on-shore school.)
Sure, I'm not really arguing that all medical schools have sterling reputations or high standards, just that the joke, as a way to illustrate the problem, unfortunately obscures the real problem. It's pithy and punchy, which makes it good joke and easy to repeat. It's sort of a peeve of mine when I see well intentioned but slightly misleading memes propagate. They're hard to fight for the same reason they are memes.
2. Humanity, where we've institutionalized pettiness, and not just once, but throughout history. :/