If the journal you work for is behind a paywall, why do you do it?

I mean it as an honest question, I've always wanted to know why someone would work for free for an organization that works for profit. I understand why the academics submit their papers; it's necessary for career advancement, but why do people review for free?

Good question. There are some institutional pressures--the person who lobbied for your appointment happens to be an editor of a paywalled scientific journal publisher. It's also a way to learn about the literature. But increasingly I'm turning down requests to referee for such publishers, in favor of open access journals and conferences. The opportunity cost in refereeing a paper is a subsidy to publishing companies--one I can't afford to pay.