A curfew is a curfew, Maybe if they had spoken to the airport auth they would have got an extension or been told not to risk it. Is it a case of a low cost operator taking risks so they dont have to pay compensation for a canx flight? Is it just the airline that would get fined or would others? At ZRH, if a a/c breaks the curfew, the ATCO is fined $14000 for allowing that a/c to do so.