When a horse and rider team start their distance at the crack of dawn, the rider is doing her best to focus on the course that has been laid out. Management has gone over the course the night before, and every turn is usually marked to help riders and horses navigate the hundred miles that lay before them. Going off course means that your horse has to travel even farther than planned. Realizing that you've gone off course for even a mile can wrack a rider with guilt.
Do I feel sorry for the New Zealand horse who had to be euthanized for kidney damage? Of course, but that doesn't mean the entire race should have been cancelled while horses were still on course, still going strong.
I am more than a bit disappointed in the FEI, and I can guarantee that those riders who took excellent care of their horses are more than a little disappointed too.