Certainly an interesting thread. I am a heli pilot and horse breaker. Firstly I would like to dispel one myth. Horses are only unpredictable to people who have little experience with them. They very quickly calm under any circumstances which do not cause them harm. Take for example police horses who deal with violent crowds. In fact up until a hunded years ago horses were involved in all aspects of industry, farming and war around the world. It is unlikely the horse spooked at a car unless the car was flying around making loud thumping noises at low level.
On the other hand, as has been stated, riding does bring inherent risk of accidents (as does walking to the shops). The only question I see as being relevant is should the helicopter have been there? If it was not taking off or on an approach then the pilot may have some serious questions to answer. Even otherwise if we as pilots have any care for public relations we had better take as much care as possible not to kill people. As with riding, it is our responsibility to limit in all situations the inherent danger to an absolute minimum.