Unfortunately the 'Market Rasen crowd' are giving the wider horse riding fraternity a bad name.
I ride horses and also work at the home of the noisiest of helicopters. The school where I ride has a land away area (is that what they called?) within 1/4 of a mile of it which is quite heavily used yet when I asked at the school if they have any trouble with helicopters they told me they weren't a problem.
Yes it was tragic that someone was apparently (I say apparently because I believe the RAF were entirely a scapegoat) killed by a low-flying helicopter spooking their horse but as already mentioned horses can be spooked by such things as carrier bags flapping on some barbed wire. As a rider you should always be both appropriately dressed to take the fall from a horse and also on your guard for the big, powerful animal you are riding doing something unexpected.