How about we skip the 20-questions and get to the point that I know more on how to legaly (or intelligently) throw a modern public aviation and/or acrobatic demonstration event. It's the same reason why you need a permit from the city to have a festival or protest in their park - not because the land isn't public, but because they don't want to be legaly liable for everything from here to the moon that may happen there unless they give it their official stamp of approval and thus take on that responcibility (and, usually, after you proove to have your event insured/bonded by some other means than the city's coffers) I don't know it all myself, and never have done it myself, but I know only the illegal or stupid hold one with thousands of spectators (paying or free admission) without any liability insurance.