Some restaurants are conveniently located next to a gym or "Poké Stop" (a designated public space where you can find items and rewards related to the game), and some happen to be gyms or Poké Stops themselves. More people means more business. And while locations can't control when, or even if, they are selected (as of right now, the app is the only entity that can control when a location is a Poké Stop or gym. For example, bougie NYC eatery Balthazar is a Poké Stop -- it definitely did not sign up for that.), players can purchase a Pokémon lure to place on a gym or Poké Stop that does exactly what you think it would: draws Pokemon to that location.
So, if your pub happens to be next to a Poké Stop, you can buy a lure, bring the Pokémon, and suck up all the spillover traffic. It's like shooting Magikarp in a barrel. And much cheaper than paying people to give your restaurant five stars on Yelp!.
As capitalism is our great nation's main export, people are starting to figure this out: