I agree that it's not polite, but frankly I wouldn't get too upset. A pack of fast cyclists on the road is like an oversized piece of farm machinery, a train, or extra-extra-extra-long trailer, it takes much more time/distance to stop safely and longer to accelerate clear from a stop, so the riders at the front who have could conceivably stopped to allow OP to turn might have made the right decision to protect the safety of the 40 riders behind them (and keep the total net disruption to all traffic on the road to a minimum) since they probably weren't planning to stop there.