For sure a scam and not related to Yelp. You can easily see any and all reviews as a member of the public or as the business owner. There are no reviews in waiting that are not visible.
Yelp does have an algorithm that hides reviews they don't think are real, though. It can be super frustrating, but the longer we've been on Yelp, the less frustrating it has been as I have seen how it plays out. Typically the reviews that get hidden are people who have little presence on Yelp, are close contacts to me as the owner (clients who are also friends on other social media platforms), people who have written reviews from the same IP address (had a husband and wife both write a review of our business, and both were flagged).
In the end, it seems fairly legit, but it was frustrating as hell when we had little Yelp presence and what we knew was a legit review would get flagged and put in the questionable section. We started paying Yelp in order to host a video on our page, and we actually had an uptick in flagged reviews after that. Seems to totally counter the "pay for play" accusations with Yelp because we were actually getting worse treatment once we started paying them.
Yelp does call frequently to try to get you to increase your marketing presence, but they are the least shady of the sites that solicit us as a home services company. Angies List and Home Advisor are the worst, as they are really gross and disingenuous with the way they do business.
Yelp has been the primary accelerator that has taken us from a 1 person company to a 25 person company, so I obviously have little to complain about.