Ros was never really under Baelish's protection, merely another of his workers. Towards the end of season 2, Varys employs Ros as a spy for him in order to follow Baelish's acts around King's Landing.
By the time of this episode, Baelish finds out who this traitor is and gives her back to Joffery for a little...fun as one may call it.
The conversation between Baelish and Varys which unravels the plot is as follows:
Varys: Thwarting you has never
been my primary ambition, I promise you. Although who doesn't like to see
their friends fail now and then?
Baelish: You're so right. For instance, when I thwarted your plan to give Sansa Stark to the Tyrells.
If I'm going to be honest, I did feel an unmistakable sense of enjoyment here.
But your confidant, the one who fed you information about my plans, the one you swore to protect...you didn't bring her any enjoyment. And she didn't bring me any enjoyment. She was a bad investment on my part. Luckily, I have a friend who wanted to try something new. Something daring. And he was so grateful to me for providing this fresh experience.
The confidant here being Ros and the friend being Joffrey .