Dudeness - they also have t think of people on the ground who may not have signed up to having an 80 year old 'plane flying over them and depositing large amounts of metal over the vicinity
Personally I'm with Terry Pratchett on this in general - you have freedom of choice but that includes the requirement to take the consequences of your actions. If I go to an Airshow I accept the risk I may be killed by an aircraft crashing - but that's a long way from, for example, the people driving on the main road and killed by the Hunter crash in the UK a couple of years back...............