It is a now-accepted and not-uncommon kind of event--no more legally exceptional than skiing, rafting, horseback riding, rock climbing, or any other moderately risky sporting or social events. Therefore, the employer would not be liable for ax-throwing, etc. injuries any more than it would be liable if it arranged a rock climbing or surf lesson outing for employees. While it may be risker than seems wise or appropriate, it  is not so risky or unusual as to impose liability on the employer.