I ride 100s with lights. However, the terrain we are riding through here on the east coast is quite different than that of the Tevis trail. I don't care how full the moon is, you aren't gonna see jack in the middle of the deep woods at the Old Dominion. And unlike Tevis, NE ride weekends don't vary by the moon phase, so I have ridden 100s with no moon at all, regardless of cloud cover. I agree the horses can see much better than we can, but that isn't helpful in the case of a low-hanging branch the horse will get under but the rider will not or the horse adjusting it stride to navigate an obstacle they can see but the rider is left behind the motion, flopping around on the poor horse's back. It has nothing to do with not trusting my horse, but in wanting to be the best rider I can be at night.
Personally, I ride with a couple glow sticks at the shoulder and a not-very-bright small lantern on the base of my breastcollar. It is enough to illuminate the trail about 3 feet in front of the horse so I can see trail changes. I also use a headlamp on the red setting to help me avoid branches. It does have the option of a white light, so I can turn it on briefly to check markings if needed, etc.
I have found this set up keeps the light below the level of the horse's eyes and in their blind spot for the most part, yet it gives me enough to react to trail changes. I have never had a horse react poorly to this set-up, including ones not my own that were never introduced to this set up until at the ride itself.
I also get terribly motion sick riding at night. I take medication to help reduce it, but I know it's just going to happen if I ride after dark. The diffuse light of the lantern is much better than a solid point of light moving all over though.
I find it disheartening that people would rather create a rule than train their horses. If your horse doesn't like lights coming up behind it, then work on its training until it does! If my horse doesn't like being passed during the day, for example, nobody is going to make a rule saying that isn't allowed.. it is on the rider to train their horse to accept the things that can be expected to happen in the ride. Those people better never come East and ride the Vermont 100, as every
runner (and there are hundreds of them) on trail has either a headlamp or a handheld flashlight or in many cases both!