The problem with free review sites is that once you've built up trust as an honest, objective reviewer, the most effective way to profit on that trust is to violate it.
This may be true. But that doesn't mean the best strategy is to completely burn your reputation. Even if the only way to profit is to sell-out, it's usually better to sell-out slowly and collect much more sustainable revenue over a longer time frame.
There are a lot of equilibria where most free review sites are mostly, but not completely, trustworthy. With enough review sites in that model, an end-user can effectively triangulate the objective truth with arbitrarily high certainty.
