Polymarket didn't serve the US last I checked, unless they've changed recently.
Note that Polymarket's non-US business is crypto blockchain whatever so ... good luck with that. The ability to get your money out could disappear at any time and as far as I know there's no recourse if they just decide to screw you over. Not that CFTC is super effective but it is something.
So is Kalshi. The entire question is how the CFTC made up the authority to regulate gambling out of statute that only ever mentions exempted commodities.