Well, buddy, welcome to Capitalism!
Get your site cranked up, program some porno price and review search software, round up some miscreants with at least minimal verbal ability and a 5th grade education, and go to town! Seriously, porn ain't going anywhere. If ADT can be "worth millions" (which I strongly doubt), you can, too!

However, if you were concerned about your reviews generating bucks that you won't see, why did you submit them to begin with?

The other issue, as I see it, is that many reviews are of such poor, amateur quality that they could ONLY exist as unpaid fodder on fan sites. From what I've seen across the Internet, most so-called "porn critics" have such poor command of basic grammar and vocabulary, let alone logic or rhetoric, that they would never pass muster at a better-than-average school newspaper --- let alone mainstream publications, (I'm allowing for typos and forum shorthand. Just saying a lot of people passing themselves off as the Roger Eberts of Double Anal Funtimes would go hungry in the Non-Porn World). So, why should sites pay for crappy reviews that would be rejected in a more professional arena?

Just raising the issue. I think your question is a fair one to ask.