The practice of slavery in the U.S. was long-established and well-known, still does not make a practice right and there are U.S. residents who do not tip as a matter of practice which they should be entitled to do. There is a legal argument that it could well be extortion and the Skycap practices could be subject to the RICO Act but it's a question of whether any individual is willing to take the necessary steps to bring such action.