>Some level of government purchases an agricultural easement.

Now you have it right. It is not the land which is purchased but the easement on the land. Ownership does not change.

One thing which could be done better? Enforcement of the terms of the easement. Right across the street from Cumberland Valley High School in central PA, a conservation easement was placed on a historic horse farm. Years later, the property was sold. And warehouses are now under construction on that very property.