As slinkyunicorn has said it's the hygeine,(health and safety act)for instance rats (which are everywhere whether we see them or not) regularly crawl on rugs, even when they are on the horses and they can carry a variety of diseases, weils being the main nasty. If we handle rugs that haven't been washed there is a chance we can contract weils through a cut on our hands.Also, even rugs that appear "clean" usually have some dirt on them as well as a horse smell, this dirt even a tiny amount gets on our sewing machines and into the workings, it acts as a grinding paste that wears the machine out as well as making them smell of horses. If we are using the machines for new work as well as repairs we then transfer the dirt and smell onto the new stuff!
THAT'S why we always insist on having rugs washed'
Oz