Minimum Stays on UK Sites

I've just attempted to book an 'affiliated site' for 5 nights during half term in June only to be told that the site is imposing a 7 night minimum stay during this week on bookings made before 30th April.  Am I wrong in being somewhat miffed by this?   Isn't this  against the ethos of caravanning and the freedom that caravanning is/should be all about? 

I can understand that some independent commercial sites may wish to maximise their income and impose such rules and I therefore choose not to give them my custom, but this is a club 'affiliated' site.  If the site wishes to act like a commercial site and not follow the club booking policy, why should it enjoy the benefits of being affiliated to our club? 

Incidentally, I would have very much liked to stay for 7 nights, but school term times and a wedding has somewhat restricted us!