I love how that affiliate site lists studies done from (apparently) 2000... you think they'd update their figures a bit since the poker industry has skyrocketed over the past 6 years... It only benefits them to make it seem more viable...
Anyway, I don't think there is a problem with poker affiliates in the US. This was asked in another thread. Online poker is not wholely illegal in the US. In fact, there are many online casinos and poker rooms that operate in the US. Full Tilt Poker is one of those, owned by several major pro poker players who live in the US. They operate under the Kahnawake Gaming Commission, which is an Indian tribe with casino rights in the US. There are several others that operate under the same license. So, speaking on a conservative legal level, I would say that if you setup affiliate programs with those sites, you won't have any worries as long as you claim your income on your federal taxes...
As far as casinos and poker rooms outside of the US, it is illegal for the owners to target or essentially allow US residents to gamble on their sites. I don't think this is an issue of morality and the evils of gambling, as much as it's just a case that the US aren't getting any taxes from this. There have been cases where owners have been arrested, but only when they came to the US, because the US has had little luck attacking these owners outside of the US. Is it illegal to participate in an affiliate program for a casino or poker room that operates outside of the US? I don't know...
Probably... But I've never heard of anyone being arrested for it, and I'm sure that's something that you would definitely hear about, because online gambling is a major issue right now. I think as long as the government gets their cut from the income you make from it, they're happy...
Who knows, this is an issue that will definitely play out over the next year or two as the government is attempting to pass a bill to make online gambling illegal in the US, but at this point, I think it's a murky area and you'd really have to do something to piss the government off for them to even notice you...