I dunno, Because they want to? I know its annoying, but unless the trail says that there is no horses allowed, you can't really do anything about it. Next time you see someone on a horse, talk to them, be nice, ask them respectfully if they can ride on a different, more horse specific trail. Tell them that it would be safer for horse riders and MTBers as it lessens the chance of anyone hitting one another.
horses, hikers and runners all have the same trail rights as mountain bikers. where im from we take a sting of pack horses and use them to build new trails, horses can pack out a sick new single track pretty damn quick. besides, a little horse shit in the trail aint that bad. sack up and respect other trail users.
As a horse rider, bike riders tearing unanounced down trails can be annoying. At least a bike doesn't have a mind of it's own. 600Kg of frightened horse is not fun to sit on. Luckily this doesn't happen often and I've never experienced a collision. As I see it, unless someone is specifically exculded from an area (usually via those charming signs), any trail is fair game. Obviously, if someone on a horse approaches an area that is visibly teeming with bikes it would be very unwise to ride into that area. Likewise, if you see fresh manure on a trail it would be advisable to at least check ahead of you before taking off at high speed. I think it's just a matter of everyone acknowledging that other people of other sporting inclinations use these trails, and keeping an eye out.
in fance we have too much trouble with trails or tracks rights (motorist noises too) some tracks are forbidden and mountains too... so it's easy respect all users on trails and be careful with horses shit!!
ya as long as the horses dont ride when the ground is wet that would be a start and also if they could try and take a shit OFF the trail i believe mtb and horse might have a chance to come together. maybe