Wow, it is really remarkable how stupid people are. The stupid old bat should not have her grandchildren hand-feeding horses, no one should be hand feeding horses. That teaches horses to bite. I have had horses for sixty years- and not one of them would bite——because no one was allowed to hand feed any of them, and if one did nip at a person, they soon believed that was the last thing in the world they wanted to do. Most bad behaviors in horses are taught to them by people—like people who feed them carrots or sugar out of hand. Horses are not stupid, that is usually a human trait.
My dressage instructor once told me: "be careful what you teach a horse because they will you it against you."
You are exactly right about hand feeding horses, it teaches them to bite and leaves them frustrated if there is no treat to be had.
Biting is a natural behavior, it is how horses communicate with each other. The trick is teaching them very clearly, no horseplay with people.