I don't think this is true in general. Certainly, they aren't paid by the publisher, but I'd imagine most referees are in paid employment with institutions that expect them to review the occasional manuscript as part of their job.
People do it as a service to the community. And I don't necessarily ask to be payed for that, I was just answering the point about "I want everything to be free all the time and never pay for anything". We pay with our work to referee the papers. Sure there are extra costs to host the papers, typeset, but they should be nowhere near 2.5k GBP per paper.
At any rate, the last time I was employed as astrophysics faculty, my offer letter told me that I should “engage in scholarly activities”. I certainly interpreted that as an invitation to review the odd paper on the university's dime.