It is a very common practice, to tether horses out like that, in England and Ireland. This especially among some Gypsies, who often graze their horses on roadsides or even the land of others. Never seen one with that kind of string, but have seen tons with neck collars on and long lines to a stake.
In the '70's, I knew an Arabian Horse breeder in the US, who also used to stake his horses out in a similar fashion. I never liked the practice and wouldn't dare use it on one of my horses, but the Arabian breeder's horses, never seemed to get into trouble.