This is an entirely untrue statement of the law. You're missing one major point about unions -- they provided certain protections under Section 7 of the National Labor Relations Act for "employees." Poker players are not employees of the WPT, or (with the exception of some house players, I suppose) of any casino for that matter. Federal labor law just doesn't haphazardly protect whoever wants to form a union, and not everyone can form a union. Indeed, I would argue that because poker players are independent contractors, they legally cannot form a union. Thus, they are limited to putting t