Many of the island's stranger quirks are made accessible to an international audience by relating them to more conventional customs. Here's a darker example:
I would ask you to bear this in mind. Colonsay has an ancient feudal society which basically wants to go on being feudal, provided they can find someone who wants to plat at -- and finance -- being a feudal baron. The term "laird" is slightly fey and old-fashioned. I am the landlord and the proprietor. These are the facts. I let houses to the other people. The island is my property. 
Aside from being a commentary on modern business practices, the laird pretty much sums up the island's situation -- a fantasy world from the wrong century. Everyone knows this, yet nobody really seems to care, much as they don't care about the island's myths. The dying small town is a common American story, but here it isn't so much of a tearjerker. Does McPhee want the reader to care? If so, to what end? If not, why bother with the decline of the island at all? Is it just another American convention in Scotland?