ISR, I appreciate what you are saying, but many property owners that allow horses to be ridden on their land/at their facility deal quite comfortably with the “floodgate of other questions.” First of all, a helmet is a very standard piece of safety equipment for many sports, and I think that not wearing one could easily be viewed as egregious by a landowner, as in, “I can’t believe I allowed those people to gallop their horses all over my property and they weren’t even wearing helmets. It’s no wonder an accident happened.”
As far as the other questions, if someone shows up for a hunt mounted on a horse that is behaving unsafely, or a rider is obviously over mounted/out of control, I think that it is the responsibility of the master to excuse that person from the field. For example, I as a BO would intervene if I saw a dangerous situation unfolding on my property, and similarly, because our landowners aren’t generally present when we hunt, this responsibility falls to the master instead of the landowner.
Also, in the US, liability is generally lessened if proper safety precautions have been taken.