Pre-med is an aggressive weed-out process everywhere. Even schools with the highest grade inflation like Stanford have pre-med courses where fewer than half of students earn A- or higher grades, while highly selective state flagships may have only a quarter of students earning A- or higher grades in pre-med courses. See http://talk.collegeconfidential.com/pre-med-topics/2074436-some-colleges-have-grade-distribution-information-available-by-course.html .
The difference is that a non-pre-med biology, chemistry, or engineering major will usually see B grades as good grades (unless s/he is trying to get into a capacity-limited major which admits by grades/GPA), while a B grade is a bad grade for a pre-med. I.e. at many schools, B grades will not weed out a non-pre-med biology, chemistry, or engineering major, but B grades will weed out pre-meds.
Pre-med grading scale:
A = acceptable
B = bad
C = catastrophic
D = disastrous
F = [foul language]