It's not illegal for US people to view it and use it as a poll. Afaik polymarket doesn't currently charge fees, so they are trying to become a well known/respected alternative to official polls, economists etc on debatable topics.

Seems ok to me that they want to build their brand in the US and assume US people will bet on other topics and take interest in viewing the election results.

I mean I also think it's an open secret that most or at least many of their users are probably US using VPNs. But I don't think this necessarily changes anything about their official goals