Check out the recent reviews on Yelp. You will find that even people who have complaints are giving 4 stars! Gotta get those Community Rewards prizes!
Here's what Yelp says about a company asking for reviews (click on graphic to make bigger):
Yelp on attempts to mislead the consumer:
Note: "For example, some businesses try to bias reviews in their favor by incentivizing people to write positive reviews."
What to do if you think a review has been solicited:
More on flagging a review you think has been solicited: