Probably because the desire to be at a racing event is such that you're even prepared to pay for the entry privilege, hence the 'at your own risk' disclaimer.
The cadets would be there because that is what is demanded of them, and as such have no 'opt out' option, even if the danger element is brought to their attention. Therefore only the foolhardy would sign a disclaimer saying that if their 'employer' indirectly injures them, it's their own fault for taking the job in the first place!