Someone already posted The Dodd-Frank act, which is the main reason you can not operate over-the-counter derivatives that are not regulated by the US authorities.
On top of that, the U.S. is one of the only two countries in the world where taxes are based on citizenship, not a place of residence, Offshore CFD Brokers will be very reluctant to offer services to US citizens (and they should), as they will not be able to declare any substantial amount from the broker. You don't really want to be messing around with Uncle Sam. If a broker gets restricted by the authorities of any of the 194 countries around the world it might have an impact. but if you break Uncle Sam's ba#*s, as a broker you are pretty much done, all the important liquidity providers are based there, server infrastructure, payment service providers, etc getting sanctioned by the US means you don't have friends anymore in most of the financial regulated world, for a business that constantly needs to send funds around the world, is an instant Game Over, even if they are not operating in the country.
On top of that, the U.S. is one of the only two countries in the world where taxes are based on citizenship, not a place of residence, Offshore CFD Brokers will be very reluctant to offer services to US citizens (and they should), as they will not be able to declare any substantial amount from the broker. You don't really want to be messing around with Uncle Sam. If a broker gets restricted by the authorities of any of the 194 countries around the world it might have an impact. but if you break Uncle Sam's ba#*s, as a broker you are pretty much done, all the important liquidity providers are based there, server infrastructure, payment service providers, etc getting sanctioned by the US means you don't have friends anymore in most of the financial regulated world, for a business that constantly needs to send funds around the world, is an instant Game Over, even if they are not operating in the country.