Sports betting. It probably does cause actual harm to some, but that's debatable. And should all occasionally harmful things (like say, guns and vodka) be outlawed? Liquor stores and betting shops in poor neighbourhoods: are they harmful? Should they be outlawed?
What this company does is legal many places, though not in the USA. Were some of the people who chose to use the site located in the USA? Yes. Is the site merely a front for enabling US citizens to break their country's law? Harder to say what the percentages of users are.
This seems similar to closing down a betting shop located in Canada, on the grounds that a lot of the patrons had crossed the border to go there and break US law.