I believe that in some cases, the larger the size of the ebook in bytes, the more they get charged for distribution. A nice cover can eat up more bytes that all the text in the book combined. Thumbnails are small in size, so they wouldn't cost as much to include as the full size cover image. It seems to me that being cheap is what it's all about, when you don't even include a simple cover image of the book title and author's name on a plain background.
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