There could be a conflict of interest, no doubt. But I've been here long enough to see some long time recommended docs removed because their results have not been great. So while the people behind the site have a financial incentive to recommend as many docs as possible here, they must also maintain the site's credibility with the patients. Otherwise, why would a doctor want to be recommended on here in the first place?
This is actually true for some docs. Some very qualified docs chose not to be a part of the HTN for various reasons; some being the constant evaluation of their work, the pre-screening, the fee, and other things I can't think of at the moment. But that is their own choice and doesn't necessarily reflect poorly on the site.