The US companies based in the US (by the way the "UK" companies in question are not really very UK, UK, UK, if you get my drift) are subject to US law and regulation on gambling. Non US companies aren't which is why the UK cos are as they are, and why the US gov't wants to shut them out of its market if it can't regulate them. For once the USA sems to be doing something right even if its regulation of terrestrial gambling is a laughing stock the world over (except in Sicily).