I don't know anything about practice regulations, but I wonder if the insurance company reduces the premium if spectators are not permitted.

Another thought on the lines of more marshalls required for spectators in the circuit and we often hear complaints of difficulty in getting marshalls (ALL VOLUNTEERS) for two days.

Look at the situation this way the circuit is losing revenue over the weekend, so there must be good sane reason to keep spectators out.

Thoughtfully,

Big D.