a Mr Borst snip...
In many cases these margins appear to be avoided because of telephone poles. I don't know if they are official right of ways owned by utility companies, it would seem so. 

my comments...
well first off a right of way or access agreement is just that.... it allows access to the property but in no way conveys ownership to whoever is using the right of way.  this does not change if the right of way is for electric or gas.  both have essentially been obtained via public domain laws.

in some places the observation that margins on the edges of fields is shrinking is likely correct.  basically if the ground is fertile enough and flat enough and the crop is valuable enough farmer will plant up to and beyond their legal property boundaries.... quite often including the county or state right of way < if there is an active set aside/conservation program then there is some marginal economic benefit to placing as much of your input as possible on your most productive soils... this is just good and basic micro economic thinking.  In other places where farmers have concern about pest it is not uncommon to see weed eradication products (like roundup) to reduce any vegetation in these same areas.  of course if folks in an area use irrigation to any extent utilization of these margins would be more difficult.  Planting and harvesting equipment has gotten more techno savey over the years but I cannot see any reason why anything special would be required for a farmer to plant or harvest right up to the edge of the road.  flatness of the terrain would seem to me to be more important than some form of specialized machinery.

some of this may also depend on the personality of the farmer.  my wife likes to suggest that some farmer's have 'english garden mentalities' where there is not a flower out of place or a weed to be seen at any time of year.

ps... some years ago I heard something along the same lines of thoughts from an old beekeeper from Wisconsin (transplanted to North Dakota) about the presence of wild mustard and the margin of fields in the almond groves in California.  his basic view was these habitats provided a good deal of high quality pollen to hives when the almonds were not in bloom and the last time I was out that way the mustard appear to not be present and 'the english garden mentality' seem to be quite in fashion..... not a weed one to offend the eye for as far as a man can see. 

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