Simple. IT IS ILLEGAL to play at Pokerstars or FullTilt or any of the typical places from anywhere in the USA. The US Gov cracked down on them.
I quote: "Here is an EXCELLENT alternative where it is COMPLETELY LEGAL for USA players to play and win lots of real money and prizes, and never lose any money."
You may want to consider http://www.clubwpt.com/?cmpgn_id=6242&bnr_id=110
As stated at FTC: "Use Club WPT Bonus Code PLAYFREE, No deposit needed. No Risk Play System. COMPLETELY LEGAL! "
With the above link I provided, you cannot lose money, and only win money. No limit on how much you can win, but maximum you can lose is something like $0.49 per day (yes, I said 49 cents), which is what your full membership daily cost would be.
So, as you can see, or the point being, there is another model out there.