You're wrong on that I think. As IB is SEC and CFTC licensed and regulated, they fall under those rules and must follow the higher minimum net worth (ditto for any other broker). Oanda isn't beholden to the regulations of the securities and futures regulators and hence doesn't need to have that requirement.
Oanda is actually registered NFA and with the CFTC, RFED, and FDM. They're very beholden to the regulatory scrutiny of America. Do you mean for dealers offering other securities than forex?