There are 2 basic reasons that financial services might be offered differently outside the US, as compared to inside it:
Perhaps the market in another country is more heavily traded than in the US, or a firm that does well originated in another country, and is more competitive than US-based counterparts. Of course, prevalence of online trade should imply that such benefits would quite quickly be available to all global citizens.
The firm is offering services outside the US in an attempt to avoid either US regulation or US law enforcement.
So ask yourself a question: what makes your supposed 'best' forex traders better than what's available in the US? Is it easier to set up an account because fewer 'Know Your Client' documents are required? Are you allowed insanely high leverage amounts that the US government prevents its firms from predating on citizens with?
Be cautious of 'investing' your money with an online-only firm that has no physical presence in your country of residence. If fraud or malfeasance occurs, it would be difficult, prohibitively costly, or perhaps impossible, to pursue that firm for damages.