[timr]

"Now, this evidence exists mostly in hearsay-type form: blog posts, forum posts, huge reddit threads. You can find similar stuff for BBB and Angie's List. There are also a couple thousand FTC complaints against Yelp, at least."

No, the "evidence" exists entirely in hearsay form. All you ever see are stories about parents, friends of friends, distant cousins, and so on. There's almost never so much as a link to the business page. Why? Because it's trivial to debunk these claims when you can see the reviews.

"It seems silly to believe random people on the internet over tossed lawsuits and a dropped FTC investigation."

Not just silly, but absurd. It's internet conspiracy theory, and people are just thoughtlessly repeating it as fact.

"But I see this as the likely result of Yelp being cagey about how reviews and advertising interact, and being able to hide behind a policy that promises a scrupulous approach and proprietary algorithms."

If Yelp revealed how reviews are filtered, the filters would be rendered useless overnight. It would be the equivalent of Google publicly documenting their search algorithm. And while I think Yelp does many things badly when it comes to this stuff, they're 100% clear about how reviews and ads interact: they don't.