Why didn't people taking up the Ice Bucket Challenge die?
Theoretically, any treatment can cause death. Drinking water can be lethal and oxygen is highly toxic and a strong oxidizer.

To quantify toxicity we usually use Median Lethal Dose (LD50) metric. That is a dose at which half (50%) of test group is going to die.

To answer "Why didn't people taking up the Ice Bucket Challenge die?":

Dose that people used was not high enough to cause any significant increase in mortality. Maybe some people did die, but it was just masked by other news (to some degree)