Yelp is adamant that they don't affect reviews based on payment. That said the order and selection of reviews on preview pages in terms of positive vs. negative are based on how much a business pays.
Honestly, I'm sympathetic to Yelp. The money has to come from somewhere. And there's at least superficial evidence that you cannot operate a honest review platform at a reasonable profit rate (e.g., glassdoor, consumer reports).
It can come from advertising in-app/sponsored results appearing at the top of search. That's not extortionate, and doesn't compromise the whole purpose of using the app.
Either the ROI on those ads is terrible (Thus leading to low-margin businesses not advertising on Yelp), or they want more money, at the expense of long-term confidence in their website.
It's possible that the current, bloated version of Yelp, with a headcount of 6,000 cannot survive on search ads revenue. [1] It's also possible that a smaller, leaner version of it could.
[1] Facebook, by the way, employs ~40,000 FTEs. Facebook's revenues are 70 times that of Yelp's.
Honestly, I'm sympathetic to Yelp. The money has to come from somewhere. And there's at least superficial evidence that you cannot operate a honest review platform at a reasonable profit rate (e.g., glassdoor, consumer reports).