Is it normal/ethical for student grades to be assigned according to quotas?
Unfortunately, the universities and colleges are run by managers and not only by academics. In ideal world, all students' work should be marked according to its merit; for example, if all mathematical problems are solved correctly, then the paper deserves 100%. Academics usually understand and share this view (apart of a few strongly believing that "no work ever deserves 100%"). 

Unfortunately, people who manage universities, are not always guided by common sense or mathematical reasoning. They seem to invent some obscure metrics and force them on the rest of academic staff. Academics do not think it is correct, but they have to play by the rules which are set by non-academics. 

Unfortunately, students become a collateral damage in a clash of two cultures: academic and management. If you want to query this, best ask not your professor, but a Dean (anonymously or as a part of larger group). The misguided policies arrive from the very top; your professor has not much power against it.