In recent chapter, we learned that WW was one's very interested in Sun God Nika, who was supposedly be able to free all slaves. If WW was only a prisoner he would not be so obsessed with that idea. He is not the only strong character who was turned into a slave - Fisher Tiger and Kuma are good example of that. Other thing that could be adding to this theory is his eye tattoo. It could look like Dragon's Hoof.