In this case Bitmex was not incorporated in the US, informed US residents that they were not allowed to use the platform, and even blocked all US IP addresses from accessing their services as well. As the article mentions, yes, some people got around this with VPN blocks, lying about their location, and more. Apparently the US views the solution to this loophole as arresting the founders of Bitmex, regardless of their country of residence, for not complying with US laws. As another commenter reminded me to mention, Bitmex also did not touch USD: they didn't allow withdrawals, deposits, nor transactions in USD, ever. The discrepancy between this treatment and that which established US megabanks receive is made perfectly clear by the latter section of this article, in which billions in fines are often paid in exchange for having purposefully committed blatantly illegal activity for profits on a massive scale.
Perhaps there is more to this story that we do not know (I see accusations of a lack of solvency, front-running, and other more serious activities), but if there is no true bombshell awaiting explosion here, I certainly hope his legal defense is sufficient for a solution in his favor.