Hi Midas, brokers accepting US clients should be registered and regulated by the CFTC and comply with all requirements - some of them are significantly different than EU regulators requirements (like "first in first out" rule - no hedging, leverage regulation, different capital resources requirements and others). So for some dirty EU brokers it would not be even possible to come with their offer to the US market. For some brokers it is too risky to accept US clients until they are regulated by the CFTC. And finally there are some offshore brokers who choose not to comply with US laws and feel they are outside the reach of enforcement.
Ryan