I've been around the block with exchanges and derivatives platforms. I watched Bitmex rise from nothing to the highest volume platform in the world. They did it on the backs of US traders. For several years they just buried the ban in the terms and let us register and trade from US IP ranges. That's a pretty clear indication that the ban was "in name only" and they were intent on welcoming US traders.
Here's one recent case on Bitmex: https://www.reddit.com/r/BitMEX/comments/99yo3u/us_citizen_used_fake_info_vpn_to_trade_on_bitmex/ee77ad1/
As far as I know that's typical.....account closed, funds returned. I've never heard of them confiscating peoples money in these cases. I assume the same is true with Bitfinex because there's lots of reports of frozen accounts/KYC demanded but people don't say their money got taken.