The relationship between Arby's and Pepsi is mutually beneficial. Arby's needs a refreshing beverage for people to wash down their meat sandwiches, and Pepsi wants people to wash down food with their product.
The deal also requires Arby's to feature Pepsi in two commercials per year, but apparently, somebody at Arby's forgot.
Due to this oversight, Arby's released a 30-second one-shot spot admitting their mistake. Either that or this was a marketing stunt that had been planned for months and everybody fell for it.