From the PeerJ Blog:
Since our launch, PeerJ has given authors the option of publishing the peer-reviews for their articles (and approximately 80% of our authors have chosen to do so).
These public reviews have always had a unique URL (and hence have been citable by that URL), however we are pleased to say that we have now applied DOIs to all of our peer-reviews and we provide information as to how to cite the review.
One reason to coin DOIs for reviews is so that if the currently used PeerJ website URLs change for whatever reason then there will still exist an up-to-date mapping to the review via the DOI.
We also believe that reviewers should receive credit for their efforts.