Ultimately, the taxpayer funds the researchers' salaries, and the researchers spend some of their research time on peer review instead of advancing their research.
But these hours dedicated to peer review are not the issue: something scientifically useful gets done. It's the tax dollars that go to Elsevier, in exchange for literally nothing*, that we should worry about.
* sometimes they charge you, the taxpayer, in exchange for access to the research output
Technically, no one; in practice, it gets done by both faculty and grad students/postdocs paid by universities as something that's expected of them to keep getting their wages for their primary work.
But these hours dedicated to peer review are not the issue: something scientifically useful gets done. It's the tax dollars that go to Elsevier, in exchange for literally nothing*, that we should worry about.
* sometimes they charge you, the taxpayer, in exchange for access to the research output