How can anyone imagine that any of the blame rests with anyone but the men who participated in this? If someone is under the influence of drink or drugs, they are not in a fit state to consent to sexual activity; and if they attempt to initiate sexual contact, the only correct course of action is politely to decline. The fact that they may appear to be enjoying themselves is of limited relevance; the real question is, would they be so happy for the same thing to happen to them while sober? I have known people injure themselves seriously while drunk, and laugh about it at the time. Presumably, they would not be so happy to suffer the same injury while sober.
Leaving aside for the moment any apparent willingness as a participant on the victim's part, would you at least agree that she was deceived by a false promise? For instance, had the challenge not been something sexual but something onerous such as cleaning the bar's toilets (though it is my unfortunate experience of drunken men that the toilets are the one of the least likely places for them to be found pissing) to win one drink, would you consider this to be unfair, and that she was taken advantage of?
The blame belongs with everyone who thought this was a good idea. The organisers of the competition, and the men who took part. For the deception as to the true nature of the reward, and for the nature of the challenge.
Having said that, I am aware that mobs can turn ugly (and the inverse of the IQ of a mob is equal to the sum of the inverses of the IQs of each of its members) and at least some of the men involved might have felt pressured into participating, for fear of having their sexuality -- and by extension, their worthiness -- questioned. If they were acting under duress, then they too are victims (but not blameless ones; it is better to risk being called a poof than a rapist).
TL;DR -- this is a reminder that we still live in a horrid, sexist culture. And if some people stop and think about changing that, then maybe some good will come of the whole sordid mess.