The Dutch authorities are insisting that tickets can only be sold as part of the travel package and any unsold tickets will be returned to Alkmaar. They will not be made available to fans on the day of the match.
I wouldn't wish it on the fans who have booked but how embarrassing would it be if the club had to cancel the flight again due to lack of interest. Potentially no Boro fans (officially) at the game.
I don't know my European law but the restriction of tickets could contravene the European Convention of Human Rights.
Whatever the restriction there will still be 2000+ Boro fans travelling to Holland for a couple of days of sight seeing in Amsterdam.