I believe that some of the videos involved children residing in the US, a finding which could possibly give the US joint jurisdiction.

... deputy assistant attorney general Richard Downing said in the press conference. "The Department of Justice will not stand for exploitation of our nation’s children. Let today’s announcement send a message: If you were involved in these crimes we are coming for you."

Also the users of the site, at least 23 in the US, could possibly be considered as co-defendants in the alleged exploitation of children.

In addition to the site takedown and Son's indictment, officials around the world also arrested a total of 337 Welcome to Video users in 23 US states, Washington DC, and in 11 other countries.

However, I am not a legal expert. These are merely my uninformed opinions.