Because his service facilitates financial crimes in US and interpol is a thing? You can be extradited from any two countries with treaties.
Which seems unlikely in his case as he is based out of Russia.
US can and will restrict his access to US persons/assets (incl. vice versa).
They're doing business in the US. Only their US business is directly affected. Most crypto sites have different operations in the US because different countries have different laws, and you technically have to follow all of them.