Why? Negligence is one thing, but we typically don't hold people criminally responsible for things they couldn't reasonably have anticipated. Civil liability is different, but clearly LD's punishment isn't meant as compensation for Twilight and friends. Military and police policy is typically also not to punish people who take all reasonable precautions. Probably we're too willing to give police the benefit of the doubt when they shoot someone who turns out to be unarmed or bust into the wrong house and kill someone's dog.
There's not much of a judgment call here. It's not clear that any non-flyers other than the main cast have ever tried to go to Cloudsdale or the Wonderbolt academy.
If we assume that the Wonderbolts are military organization.
http://usmilitary.about.com/library/milinfo/mcm/bl134-42-1.htm
Punitive Articles of the UCMJ
Article 134 - (Reckless endangerment)
Elements.
(1) That the accused did engage in conduct;
(2) That the conduct was wrongful and reckless or wanton;
(3) That the conduct was likely to produce death or grievous bodily harm to another person; and
(4) That, under the circumstances, the conduct of the accused was to the prejudice of good order and discipline in the armed forces or was of a nature to bring discredit upon the armed forces.
Explanation.
(1) In general. This offense is intended to prohibit and therefore deter reckless or wanton conduct that wrongfully creates a substantial risk of death or serious injury to others.
(2) Wrongfulness. Conduct is wrongful when it is without legal justification or excuse.
(3) Recklessness. Reckless conduct is conduct that exhibits a culpable disregard of foreseeable con-sequences to others from the act or omission involved.
The accused need not intentionally cause a resulting harm or know that his conduct is substantially certain to cause that result. The ultimate question is whether, under all the circumstances, the accuseds conduct was of that heedless nature that made it actually or imminently dangerous to the rights or safety of others.
(4) Wantonness. Wanton includes Reckless but may connote willfulness, or a disregard of probable consequences, and thus describe a more aggravated offense.
(5) Likely to produce. When the natural or probable consequence of particular conduct would be death or grievous bodily harm, it may be inferred that the conduct is likely to produce that result. See paragraph 54c(4)(a)(ii).
(6) Grievous bodily harm. Grievous bodily harm means serious bodily injury. It does not include minor injuries, such as a black eye or a bloody nose, but does include fractured or dislocated bones, deep cuts, torn members of the body, serious damage to internal organs, and other serious bodily injuries.
(7) Death or injury not required. It is not necessary that death or grievous bodily harm be actually inflicted to prove reckless endangerment.
Lesser included offenses. None.
Maximum punishment. Bad-conduct discharge, forfeiture of all pay and allowances, and confinement for 1 year.