I guess they just have to be very clear about the risks when getting participants to sign the waivers and should stress the importance of being physically up to it. Clearly if there is a particular obstacle that people are getting injured on then the organisers should take steps to minimise that danger. As you say, it’s the organisers’ job to manage the danger, but not to eradicate all risk, or the event wouldn’t have the same appeal.
With any mass participation event there will be the odd injury or freak accident, even if it were just walking down the road. It’s the law of averages.