The chain itself is not an agent of soring. The chains do not hurt the horse unless A) they are excessively heavy. B) they are used in conjunction with an irritating subtance/chemical.
Heck, any liquid put on the leg will cause for more concussion, and potential for injury, because the protection and cusioning that the hair
Technically the horses do know how to work them as their intent is to make the horse attempt to step out of them, as they would do in snow or tall grass. That is the intended purpose of the chains and like with any tool, it can be used improperly. If it comes down to the chain hurting the horse, it is likely the LEAST of the pain that horse is going through.
The only chains that are SUPPOSED to hurt are kicking/pawing chains which are basically "self correction" devices, and I have seen people of every dicipline use these.
Humans and horses are built differently, think differently, react differently to everything. The "running barefoot for a mile" is a non-issue, for many different reasons. Most horse/human comparisons are nonissues, as the horse and the human cannot be compared as even remotely similar beings.