Coke probably can't afford to lose the Wal-mart market.
Wal-mart can promote the competitors, Coke has no other recourse.
Coke prices in Wal-Mart are just the same as any other grocery stores, except that Wal-Mart honors competitors' prices and there's no limits on quanitity. Coke had plenty of recourses here. They could have told Wal-Mart to get bent and pulled their products out of their stores, which would have hurt some grocery sales at Wal-Mart because people buy soda in addition to other things too.
Coke passed up a good chance to fire a shot across Wal-Mart's bow, IMO. Don't get me wrong I love Wal-Mart but I like it when businesses stand up to them like the Snapper dealer and how Netflix ate Wally's lunch in DVD rentals.