The more subtle argument is that scholars might tailor their work to make it more acceptable to the hypothetical peer reviewer, censoring themselves, or else choose less risky, less polemical stances and research programs. They might be afraid to take on established scholars in their fields. Peer review is oppressive, then, because it subjects younger scholars to a system in which they must conform to established opinion.
My first response is that this is not a problem with peer review per se, but with any kind of system for judging scholarship, as long as it's other, more senior scholars who are doing the evaluating.
Secondly, I think that young academics want to be conformist, more or less. They want to join a community of scholars. To do so they must exercise a certain tact, paying homage to the existing consensus, insofar as it is worthy of respect (even if possibly mistaken). The argument I pose above assumes that everyone wants to be a polemicist rather than to get ahead by jumping on the bandwagon. Is there anything wrong with scholarship being gregarious?
Now since I myself am a polemicist, sometimes going against received opinion, I have to say that some of my controversial work was accepted by very good journals. Hispanic Review wanted my two most argumentative articles, and not the five or six other ones I have sent them. So if scholars are afraid of being polemical, maybe they are mistaken about what the consequences will be.
Also, it seems to me that if you want to be polemical, you have to accept the consequences. You cannot argue against everyone else and then turn around and wonder why your view is not immediately welcomed. Ultimately, originality will be more valued than agreement. If someone writes a book to refute my views, as someone did, then I have to say that at least it's my ideas that are in dispute, not someone else's. It's hard to get upset about that.
The academy sometimes rewards originality and risk-taking, and sometimes rewards mere competence and conformity to received opinion. All of us are somewhat ambivalent about how much originality we really want, so that the same person who claims that peer review can stifle creativity might turn around and stifle someone's creativity in a peer review. Oppression, though, is the wrong concept to apply here. It's more a question of ambivalence, I think.