[neutronicus]

Because in a grade-inflated world, the professor who gives B's is doing a disproportionate amount of damage to his/her student's prospects. If you give a student who "does B work" by your standards a "B", you're actually denying that student the positive outcomes that would have accrued to a B student 50 years ago. Professors have to act in unison on this front, and even then, you're giving people who went to school 10 years ago an advantage.