This is a very good and puzzling question.
why is CFDs or the daddy of them all - spread betting - illegal in the US?
CFDs are effectively trading on the margin, and you pay funding. the broker (supposedly) trades the underlying for you and is hedged so you take the risk and they take a comission much larger than on a cash trade. Gains on CFDs are tax liable as well.
so what's the problem?
I'm not going to get into spread betting. sp is to CFDs/futures/margin trading as crack is to cocain. it should banned and SP founder/brokers hanged. disgraceful.