Michael, I think you are talking about two different things. As a photographer for hire, if you establish that the negatives are your property then you have the right to refuse surrendering them. Commercial photographers usually bill for the film and surrender the film at the end of the job, but these situations are confined to those people who were hired and paid for a specific purpose with all the restrictions agreed upon before the job starts.
In Robert's case it is a model who was a friend and now has second thoughts because of her new relationship. I dont see the harm in trying to meet them half way, if anything just so that Robert can maintain a good reputation, even though he has done nothing wrong.
OTOH I think your farmer example is a bit misleading, personally I always ask permission to photograph a private structure, even if I am in a public road and have every right to do so, this courtesy towards the owners has netted me more great pics than if I had not done so, recently I had a similar situation, since I asked permission, the owner actually left one of his sons with me to help me and opened up the place specially for me which resulted in two great shots, and better yet, I was invited to come back any time!
So good manners usually pay off, and to tell you the truth if I took a pic and the owner objected, I would gladly give him the neg and print.
In Robert's case I think the fiancée has just pissed him off and has become a no win situation. They were wrong to harass him and his bride at the wedding and the following days and now he has dug his heels (which is very understandable).
In the end, safeguarding my reputation and maintaining a good relationship with the locals is more important than any "masterpiece" I might have created, I figure if I can do it once, I can do it again. No picture is worth the hassle and anxiety that Robert is going through.