Yelp absolutely prohibits businesses from soliciting reviews from their customers.
Soliciting reviews can cause you to receive a “review solicitation penalty,” which can make your business locations less likely to appear on Yelp.
We strongly encourage all users to adhere to this policy. Yelp aggressively looks for any signals that might indicate that reviews are being solicited. This policy is one of the key points of differentiation between Yelp and Google.
You can read more about this in the the Yelp Support Center.
Don’t ask customers, mailing list subscribers, friends, family, or anyone else to review your business.
Don’t ask your staff to compete to collect reviews.
Don’t run surveys that ask for reviews from customers reporting positive experiences.
Don’t ever offer freebies, discounts, or payment in exchange for reviews — it will turn off savvy consumers, and may also be illegal. Yelp has a Consumer Alerts program to let people know about businesses that engage in this sort of activity. For the same reason, you also shouldn't offer incentives for users to remove reviews.