It didn't say no residential development. It said that the land was to be used as a farm. It's fairly common for a farmer to have a house for himself on his farmland.
Same difference. These conservation easements are usually very clear. They don't want the property to be used to live on, just farmed, or as a park, etc. While you and I think every cute little farm deserves a farm house, in the legal sense that's simply not allowed if the deed or logical interpretation of its langague doesn't permit.