People drop out of pre-medical track everywhere. There are only 20,000 seats in US medical schools each year .The number of college freshmen who wants to become doctors is several times higher. Pre-med biology and chemistry classes discourage quite a few, because, honestly, students are better of looking at other options sooner than later (e.g., finding that they cannot get a decent MCAT score and then trying to figure what to do with their biology undergrad).
As I said earlier, no. of applicants for medical school from Northwestern undergrads is higher than those from any Ivy (outside of Cornell, which also has a much higher total no. of undergrads), or other selective schools like Stanford, University of Chicago, Rice, or Vanderbilt. So, I don’t think one can conclude that Northwestern is unduly discouraging many that would have done fine elsewhere. JHU, WUSTL, and Duke have higher no. of applicants, but I think the no. of incoming students interested in pre-med could very well be much higher there (because these schools are most known for their medical program).