Isn't making it public against the rules of peer review?
Not explicitly. Some journals have rules or guidelines against that sort of thing, but they're not always strictly enforced.
In my own experience, sometimes you're better off leaking the information into the public domain so that your field can progress as quickly as possible. And to establish precedence, of course. Especially if you've got research that you know is important, but your reviewers keep striking it down for possibly political reasons, you're going to feel more than entitled to put a previously rejected version of your manuscript on arXiv or send something out to other leaders in your discipline.
It's a terribly grey area that has been substantially muddied by the internet disrupting traditional publishing standards.