The more I think about Boudreaux’s Minimum Grade Law proposal, the more I realize it has already been quietly enacted. 

The names of the departments vary from university to university, but there’s almost always a department or two willing to pass along a student with some C’s and the occasional B.  At worst, a few general education classes stand as the only real barrier to graduation: once you’re inside one of the Minimum Grade departments you’re home free.  You just have to ask around to find out which departments enforce the MGL. 
As an outsider, it’s hard for me to judge why the MGL is enforced in each particular case–perhaps a desire to get more students into an unpopular major, a random collection of soft-hearted professors, a desire to avoid perceived injustice–but as an outsider I can tell colleges behave “as if” the MGL is on the books.