In the U.S. things like this (eg Contract for Differences) are illegal, and the SEC regulates these sort of things.

I don't think US investors are the target here. They can already make bets on the SP500 in a number of ways: ETFs, futures, options etc. - unless you wanna make a highly leveraged bet which traditional broker margin requirements will prevent. The tradfi venues are probably even a bit cheaper (though we are talking about a few bps difference).

If you want to trade MSFT in the US just... use Robinhood?