It's a countermaneuver against grade inflation.

Students and their often overinvolved and influential parents put a tremendous amount of pressure on instructors to provide high marks regardless of performance. This was always an issue but has become more and more uniquitous in recent decades.

Although some manage, it's extremely hard for indivudal instructors to stand up for earnest critical grading in the face of all this pressure. However, an institional policy like this lets them point to that policy as a sheild that deflects responsibility from individual teachers to a faceless, indiffent bureaucracy.

That's not to say that this is the best possible such countermaneuver, but that's the role it's trying to fulfill.

The grading system is already long broken -- far removed from your own meritocratuc ideal -- and this is a meager attempt to do something about it.

> I've always wondered what the steelman is for curve grading.

assuming that by "steelman" here you mean "the justification", i believe the point is that a curved grade shows how you compare to others. the idea is that "getting 40% of the answers right" is meaningless if you don't know how hard the test is, so you'd rather have a grade that says "top 5% of the class".

this what i see as the justification, at least. not an endorsement of the idea

It's kind of curve grading I guess? There's no limit on A- and below, so you could have 20%(+4) A students and 80% A- if you really wanted. Or 100% Fs if you want to retire from teaching immediately. I wouldn't say I'm a curving advocate, but it seems to me 400 Calc 1 students or whatever is a large enough sample that statistically curving will not do any great injustice.