I contacted the Help Chat on this and was told that the reason students can't be assigned peer reviews without having first submitted their own paper is that doing so might encourage plagiarism. I contend that I (and I assume most other professors) would be able to identify such a situation rather easily.

My rationale for this request is that students should be allowed to earn every point possible. I doubt too many professors in an in-class setting would refuse to allow a student in the classroom to read other people's papers during a peer review session if that student hadn't submitted something.