My understanding is all financial aspects of the America's Cup are handled and licenced from a shelf company called America's cup Ltd or some such. Full ownership of the company is passed to the syndicate which wins the cup. All profits are allowed to be used in the defence. e.g. running the regatta, new boats, R&D.
The "deed of gift" which outlines the rules or each regatta ensures that the company changes hands when the cup does.