Yes, yes they did/do. It's a move straight out of the mobster playbook. It's the modern day equivalent of paying for "protection".
Yelp has also been sued by investor's for allegedly misleading them into believing that Yelp was filtering out everything that wasn't reliable, first-hand reviews from people who actually went to those businesses and paid for services.
On a similar note, I believe business owners have alleged numerous times that when they refuse to pay the extortion fees to have bad reviews removed, they suddenly get a bunch of bad reviews all at once. Also, Yelp has seemingly had a habit of allowing competitor's post bullshit reviews on businesses who haven't paid them for that "service".
I don't what currently goes on, though. Maybe they have cleaned it up? But I doubt it.
Yelp doesn't do nearly as much as they should to protect their users.