Let's sum up some "truths."
Truth: Harvard, Yale and Princeton are prestigious schools in the sense that they have a worldwide reputation for excellence in education and extremely high standards for admission.
Truth: The students who are admitted to those schools are highly intelligent, motivated, industrious, etc., and have performed at exceedingly high levels, academic and otherwise, in secondary school.
Truth: Legacies, minorities, geographic distribution, disadvantaged status, athletes -- even with those "crutches," it is still extremely difficult to gain admission because there are A LOT of qualified applicants who have some of those added boosts. (NOTE: legacies at Harvard still have SATs in the 700s and at least a 3.7 high school GPA, and usually higher in both, plus remarkable achievements in other areas. The competition among "children of Harvard alums" is fierce.)
Truth: The educational and other opportunities at those institutions is nothing short or remarkable. Astounding, actually. And not just in the classroom.
Truth: Any student who does even reasonably well in those environments, given the intelligence/other qualities they possessed upon arrival, is likely to be a person any medical school would like to take a good look at. Their job in selecting candidates to interview is so daunting, why would they NOT piggy-back on the stiff admissions policies of those 3 schools and the excellence of the education they provide.
Truth: Arguments about grade inflation, harder courses, or anything like that are not relevant. NO medical school believes that a Harvard grad had an easy time because of grade inflation and, therefore, they should take the state school student who got As with a tough curve. That would be illogical given all the other truths, and adcoms are nothing if not logical.
Truth: Adcoms are also risk-averse. They want to be sure that the students they select will succeed. I would be willing to bet a lot of money that NO Yale/Harvard/Princeton grad who entered with a B or above average, did NOT succeed, and probably excelled, in medical school.
Truth: Accordingly, many/most/all medical schools give EXTRA CREDIT to those graduates in the admission process. Of course, they must have the other pieces of the pie: research, volunteerism, good personal skills.
Assumption based on those truths: You can have a lower GPA from Yale than a person from a less prestigious school and get in ahead of that person. Not just "equal," but lower GPA -- 3.4 versus 3.8.