If you spend enough time in academic circles on the Internet, you find passionate statements that providing free peer review for for-profit journals is exploitation. I have heard this from a distinguished Roman Army scholar who has not been well-treated by his academic employers, and on the birdsite you can find things like this:
Now, in my time as a graduate student I have peer-reviewed one journal article, and reviewed half a dozen manuscripts from friends, and I have to say that the claim I am being exploited is absurd. Any wise writer sends their writing to a few trusted friends before they send it out into the world. This is such a basic feature of academic life that academia dot edu built a whole module for it, Princeton and Stanford host a series of Working Papers in Classics, and an Australian economist posts drafts of his books one chapter at a time on Google Docs with invitations for readers of his blog to comment on them. When I agree to comment on a friend’s manuscript, asking them for money would be as offensive as inviting them to my apartment for dinner and then sticking a credit-card reader in their face. Trading favours is a basic part of social relations between equals. As scholarly authors, we read other people’s work (and cite it or review it) so they will read ours. Reading yet another article on a subject is tiresome, but we do it because sometimes it will be our article on someone else’s desk when they really want to go to bed and the recycling bin is so very very close.