Under the injurous weeds act 1979 people with ragwort on their land are obliged to remove it if asked, if their land is within a certain distance (can't remember what) of a field for grazing horses. If they don't remove the ragwort, they can be reported. If it still isn't removed within the time limit they have been given by DEFRA, they can be prosecuted.

Ragwort is poisonous to horses. It is true that most will leave it alone, but it doesn't taste as bad when it is dead/dried eg. in hay so the horse may be tempted to eat it. Horses may also eat ragwort if there is no grass, or much of anything else to eat, in the field.

Ragwort has a cumulative effect on the liver, meaning you won't know how much your horse has eaten until the horse shows signs of liver disease by which time it's often too late to do anything. Maybe it's not as bad as some people say, but it's not worth the risk of leaving ragwort in your horses field.

The horse that someone mentioned ate one mouthful of ragwort and became ill had probably already eaten some on another occasion, causing liver damage, and that one mouthful was one too many.