Real Answer:
Shinto is the native Japanese religion. Shinto gods are known as "kami". One of the traditions in this religion is purification (making places very clean for their kami .) The Japanese believe that salt is the symbol of purity. Sumo wrestling has its roots in Shinto. When entering the ring, the sumo wrestler sprinkles some salt to show that he is dedicating his match to the kami. (It is interesting that the sumo ring is a replica of a Shinto shrine. The roof, which was originally held up by two posts, is now suspended from above. This way the television cameras have a clearer view of the action.) The Fathy's e-mailed the correct answer "To purify the ring and supposedly to insure the wrestler against injuries."
Here are some of the interesting answers from students: I think they sprinkle salt -