It's really not - but almost all airfields are surrounded by roads, if not residential areas or worse. The regulations have adapted to where they are, successfully, to prevent huge casualties from aircraft crashing into the crowds, but you can't avoid all associated risk. As others point out, the chances of crashing onto the A27 and not the largely empty South Downs were, by the geographic nature of it, very, very small, so you don't get much better than that without either cancelling it or having it barely visible to the audience.
However someone will have to decide if even this small chance represents a fair balance of risk/reward to carry forward.