Thanks for clarifying what happened in this tragic incident. I´m shocked with how beginners were treated in those university clubs. I won´t pretend that I understand the culture which allowed this kind of "hazing" to unpunishingly happen. I really thought that after being viciously thrown more than a hundred times, a bookish Japanese student would feel ok to respectfully bow out of the situation.
I don't know the specifics of the incident, nor do I know that much about Japanese universities, but on a more general level, I think a common feature of some initiation rituals -- both healthy and as in this case unhealthy ones -- is that the 'initiated' person often believes what's happening as well...
E.g. they may buy into the idea that the more they can withstand, the more worthy they are. And if this was a martial arts clubs with a certain 'reputation' the teenagers or young adults may have been exposed to some prior mythology about this 'initiation' and may have originally chosen to attend in some part because they themselves had bought into the idea that withstanding a lot of abuse proved something important and positive about their character -- this is just speculation but if it's the case it could make it extremely difficult psychologically ('shameful') to 'give up' or 'fail', even _without_ taking into account all the additional dynamics of the sempai/kohai relationships (which would add all kinds of other issues). Again, I don't necessarily know how this plays out in Japanese universities, but it's not hard to imagine that being a factor.