It falls into the weird category of "Special Risk" Insurance, ala Lloyd's of London, but in this case is more like a big institutionalized wager. If the Sox were 10 to 1 odds against winning the World Series at the beginning of the season, then the members at Lloyd's (or whomever) probably got a 'premium' of 1/8th the expected payout. And I'm sure both sides carefully calculated the fact that 'winners' have to claim their refund, and not all of them will (forgotten, or lost all receipts).