The theory of the online gambling and money laundry + terrorism has a very inconsistent base.
They think that if an ilegal group is in charge of an offshore online casino, could be very easy for the supporters to send money using the masquerade of electronic bets.
But as stated above, there are banks and financial institutions involved to reach those offshore accounts. It is not so easy.
In addition I think that the offshore heavens are deeply involved to preserve a multibillion legal industry.