Whether it's extortion depends on whether the party asking for compensation has an "existing right" to the action and the compensation.
So it's fine to post a review of a restaurant and accept a voucher to remove it. It's even fine to post an honest review and say "I will remove it if we resolve our dispute by refunding me my meal".
But you can't threaten them with "unless you pay me 100x my damages, I will write horrible things about you", even if they're true, since you don't have an existing right to 100x your damages.
In my book, absolutely, it is extortion. Yelp definitely uses that business model.
I would guess that anyone who goes after them will probably suddenly experience a flood of “totally legitimate” one-star ratings, and those reviews would find their way into the court case as a means of discrediting the prosecution’s efforts.
Source: experienced this sort of tactic firsthand in a similar kind of dispute over reputation.