I was wondering why after I log out of MyCoke Rewards, it logs me out of Facebook?

After entering codes and have done looking at offers and sweepstakes I log out of MyCoke Rewards site, and every time it logs me out of my Facebook account. I do not believe the accounts should be connected like that as I am usually involved doing something else on Facebook and I have to log back in and pull up that page again. Very annoying.