Well, hmmmmm I have a friend from USA who lives here in Costa Rica and every time that he uses his credit card at the casino, he write's it off as a loss on his tax papers so even if he wins he still writes it as a loss.
Therefore, if poker sites don't report each individual's win's or losses by the end of the year, NO ONE in the USA will report any wins so nothing will be taxed, there would be far more tax reductions from loss expenses.
I am not from US but this is how I understand this
If i am wrong, please feel free to "challenge the ruling on the field"