My opinion is that any slot pull winnings will be the winnings of the slot pull organizer whose card is in the machine...and, if that person is American, then yes, those winnings will be subject to income tax by the IRS. The slot pull organizer will have to pay taxes on those winnings, so it is reasonable for him to withhold funds to cover those taxes from the winnings.
Also, I speculate that the organizer of the slot pull will not want to relinquish his card in the machine to, for example, a Canadian, because he wants to further his status in the Blue Chip Club and possibly earn free cruising.