Actually the RIU is an AI resort and they don't sell food, you are issued a wrist band at check-in and they serve guests. I have heard that they sell a day pass but I can't verify that. The Anse Marcel Beach Cafe, adjacent to the RIU beach has chairs, and in my opinion, excellent food at their restaurant.

There isn't anything "formal" or in writing about having to purchase food and beverages (or at least I didn't notice it)if you pay for chairs at the Anse Marcel Beach Cafe but you can be assured it isn't typical and would likely be frowned upon. There are only so many chairs on the beach and I'm pretty sure that the business is structured so that visitors have lunch on premises. We were surprised at how many people came to the cafe just for lunch given the area is a little remote for business people.

The area has a very French feel and we really enjoyed the day there.