The real problem lies with the fact that US residents cannot trade many products such as foreign cash index options, CFDs, and to my knowledge single stock futures? If this was not the case then the change in definition would have never impacted the ability to trade those products.
But US residents are allowed to gamble in Vegas and lose it all, mortgage their equity to the hilt, play around with cryptos and all other sorts of other gambles. Yet Korean index futures and options are apparently too risky to allow US retail traders to engage in. Go figure...