I think that is a real shame.
Renewal has always been Dec to Dec. However, in recent years it has been modified slightly not to unduly penalise people who join late in the year. It is my opinion that August is not late in the year. I appreciate that you have paid a full years subs for 4 months benefit (which amounts to mere £6.25 per month), even though you have been riding with the club since April, but I feel that the opportunities offered by the club, not to mention the knowledge and support offered by other members is excellent value.
I do note you mention that it is the principle, not the money - and I am sorry you feel that way. I hope you enjoyed the time you have spent with the club, and wish you all the best for your cycling in the future.