As long as there is a US headquarters then they can be sued within the jurisdiction of the US. I am not sure where Sun is operating out of, I heard San Francisco but Steemit's corporate office was in NY, I don't know if he moved it since the purchase but they only have to file the papers in the jurisdiction where the company is located. Then considering it's the SEC or FTC there may be a wider scope yet where cases can be filed. I am sure they spoke with an attorney who specializes in these things before proceeding.