Horses running in a field do it for short periods of time of their own violation. They do not have the added weight of a rider (which light or not still effects their movement and balance).
Riding on a track they are put into a fitting schedule and run for longer periods of time, pushed on the track, carry a rider. Horses jump things out in the field, but you wouldn't be jumping a two year old. Just because they do it when they are out playing doesn't mean it's a good idea to push them past what they are physically (and mentally) ready for.