Informed risk
If one attends an event that has an eliment of risk it should be obvious enough to all that you accept these risks as part of the activity.
It is also a responsabilty of the organization of the event to minimize the risk to those attending, if you go to any Motorsport event held on closed roads ( the Isle of Man & Northern Ireland for example ) you will find some areas such as the outsides of corners are prohibited to spectators. This is a responsable way to mitigate the risks.
The air show organization in the UK is also run along these lines with strict rules about flying over and towards the spectators. Those who choose not to pay the entry fee and view the airshow from a protected area must except that they do not enjoy the same level of safety as those do.
Had there not been spectators on the road outside the airfield the casualty numbers would have been much lower and confined to those who happened to be by unfortunate chance be on the road at the time and did not deliberately choose to take the informed risk of viewing an airshow from a place with less protection.
I am sure that the review of airshow safety that will follow this event will look at risks to those outside the airfield area who have nothing to do with the event but it is difficult to protect those who choose to stand outside the protected areas but very near to the display while viewing the airshow.