Anastasia is a good obedient woman
Anastasia knows that she needs Christian more than Christian needs her. If she didn't go along with his kinky games, she'd be out the door and there'd always be another woman to replace her.
Christian has something that many men don't have, and that is a billion dollars in his bank account. She, on the other hand, has the same thing that most women have, and that is a desire for the reward that comes from being an obedient woman.
She is the one who wants to obey, not Christian.