I'm out of it a bit for reformatting for Sony so I must leave that alone for now (though you might also check the PRS section for help on that). The only thing I can say is that if you import the cover without it being inserted into the ebook file, Sony will just treat it as a picture. As far as I know you will need to manually add the cover into the file which will be difficult (if not impossible) if the format of the book is LRF.

If too have gotten more than a few digitals with no real cover so I understand what you are talking about. My guess is that it really devolves to licensing. I would hazard a guess that the publishers had a deal paying for the printing of hardcopy books but not for electronic distribution. If so then they had the choice of renegotiating a new deal or just skipping it entirely.

After looking at the plain cover that contains nothing more than the title and author over the watermark icon I'm sure you know which they chose.