I grade on a curve because if I, a grad student, graded my freshmen against my own idea of a well-written paper, there wouldn’t be any As and precious few Bs.
These students were placed in my class because they started off at roughly the same level – otherwise they’d be in remedial comp, or advanced comp, or they’d place out of comp entirely. So, given that they all came to the class with approximately the same level of ability, it seems fair to grade them on what they do with that ability.
Of course, I teach a subject where there are few hard-and-fast answers; in science or math I imagine it’s another story altogether.