It is not easy to overturn a law in the courts, so any minor technicality can get it thrown out.  Sometimes it's been the small size of the claims and the timing of arguements being made late in the appeals process, so just because the old law is upheld does not mean it is not on thin ice.  Laws are better made in the legislature.  The court mostly tries to enforce the laws on the books as they are.  

One school of thought is the responsibility to build a cattle tight fence ought to be tied to ownership of the land.  That seems to be your school of thought.  That's been the tradition.  It works well when both parties have use for the fence and share in the cost.  I do not have a problem with it at all if both parties have a use for the fence and share the cost.

My school of thought is the responsibility to fence in one's own cattle ought to be tied to ownership of the cattle.  That would make much more sense here where over 90% of the time, one of the neighbors does not have or want cattle running on their own land and would otherwise not be building a new fence.  (Again, in cases where cattle are on both sides of the fence, the cost would be shared.)  Most of the existing fences here have not been used or maintained for decades.  The newer law in Missouri makes a lot more sense to me.  

Most of the time in my neighborhood, the current law forces money out of a neighbors pocket to pay for something they otherwise would not want or need any more than a wall of flaming pink flamingos.  Current Iowa fence law makes it so, and none of the comments made here in all of this discussion make that right in my opinion.  We will have to agree to disagree.