while listed on the exchanges, its a foreign registered company. as such its stock is treated like any other u.s. listed company. i.e. your buying a product sold in the u.s. and your selling a product you bought in the u.s. if you bought the stock on a foreign exchange, then a whole different story.
Some ADRs, even though they are traded on US exchange (Ex - FTE) are still considered foreign stocks and as such dividends are subject to withholding rules of the country or origin, unless there are tax treaties in place like with Canada and UK.
So I believe you are saying that NAT is not subject to withholding as they are not an ADR?
Norway does withhold taxes on US investors, Bermuda does not....