I don´t know why reporting casino winnings would be an obstacle for players? Do you want to say the US taxes your casino winnings? If not then the only other reason would be to layer dirty money and conceal it´s origins and as we all know that´s not a major reason to gamble for most people.
As for codename´s comment, I think there is more truth in what she said than some people want to admit. Ladbrokes, Coral, etc. all use affiliates because a large number of players can be acquired from countries where the legal situation is debatable at best. They are taking players from jurisdictions where gambling is illegal until their first exec gets arrested in one of those countries. On the other hand I do agree that affiliates are a valuable source of highly targeted traffic that casinos can get on the cheap.