I'm not sure it's a lack of common courtesy. My wife, who rides with me occasionally but doesn't have the lifetime of cycling experience I have, makes unpredictable moves that make me cringe all the time. I've tried to teach her to make U-turns (rather than stop dead and turn the bicycle around) several times, and I think she gets it. Then, one day, she'll nervously try to do it without giving a thought to other cyclists (or cars) around her and she'll nearly cause a crash like the one you described. Fortunately, she hasn't hurt anyone yet, but it's taking her a REALLY long time to acquire the instincts we all have from years of biking.
Anyway, I'm glad the incident you described didn't involve my wife - and I'm mostly glad the cyclist's injuries weren't worse. I think 22mph may be borderline risky for a family-oriented bike path, though. I wonder if there's a way to close it off certain hours of the day just for cyclists who want to hammer their way down.