Polymarket is banned in most first world countries (including the US). It's not particularly tough to get around with a VPN, but they do enough that they have plausible deniability. I'm sure if the US wanted to they could go after them and prove that they're still letting US customer sign up, but it's not the open and shut case you're making it out to be.

Polymarket US is starting to roll out in the US and other countries where Polymarket cannot operate, but that's a separate company and they do abide by KYC laws similar to Kalshi.