Sarah
There could be a variety of reasons why the airport 'closes' yet is till open for PPR IFR traffic. The most likely one is that there is angreement with the local council over how many post 'official airport closure' flights they are allowed in a year & the best way to control this is by the use of PPR.
Had the heli have contacted them before the 'official' closure time then he might have been accepted & just at the normal price. As he didn't they are quite right to charge an 'additional' fee.
You have to consider that some of the required services (ATC, fire etc...) may be on rosta'd breaks at that time & therefore not 'legaly' able to accept the heli if the required number of people/departments where not in place.