I completely understand it is my error.
Anyway, I fly a lot on United and everytime the United ticketing agent has verified that I have the appropriate visa. I sometimes get slightly furstrated with their diligence but now I understand why. The NWA agent in question did not even bother to verify my visa.
Also, if it is not their responsibility then why would the Dutch immigration authorities fine the airline. the airline does have responsibility to ensure that the passengers have valid travel document. I understand that they are not liable to compensate me but they do have the responsibility to verify that appropriate documents are in place.