I am far from an expert however following https://www.investopedia.com/terms/s/seact1934.asp the SEC can over see over the counter securieties (https://www.investopedia.com/terms/o/otc.asp) that seems to cover financial contracts that derive their value from an underlying asset. As far as I understand Tether is backing its value with dollars and has already been in trouble over lending the money it officially used to back its cryptocurrency away without disclosing that to its users.