It's probably to ensure the TD can report to the IRS on cash winnings. I'm not a tax guy, but I think that if a club or TD pays more than $600 in cash to a player during the course of a year they should report it and provide a form. But collecting SS#s from everyone seems overkill. I would not want to be responsible for security for a lot of players' SS#s.
Do not provide your info ahead of time. Only after actually winning.
Also i would think that it would be more like gambling, anything over 1299.00 (or something close to that) would need to be reported.