Really? You don't think Johns Hopkins, Princeton, MIT, U Chicago, Columbia, Cornell, etc consider their student body to be the best of the best, and realize that they hand out B- and C to many people from the top percentage of American high schools who turned down invites to state school honors colleges where they'd be expected to set the curves?
I think they know, and don't really care. What does it matter to them that 500 brilliant young people give up on medicine? The 300 that survive average mid 30s on the MCAT, get into med school at a 85+% rate, many to top schools, and that lets them attract the next batch of gifted high schoolers!
I'd be really interested to see the stats on premed drop rates across some of the notoriously tough schools. I don't think WUSTL is known to be as tough as UChicago or JHop, and we lose at least two thirds (source: 950 people in my GenChem 1 class, 270 took the Orgo 2 final).