It's impossible to block U.S. players who want to gamble using Bitcoin. U.S. regulation on gambling is at a bank level. The popularity of Bitcoin gambling definitely stems a lot from the fact that U.S. gamblers cannot use their U.S. banks and debit cards to online gamble. There is also somewhat of a gray area in general as banks and traditional financial networks are not involved in Bitcoin gambling sites, just as taxation is still undefined for many.