As a test, I decided to load iTunes and buy a $0.99 book called Wine For Dummies. I got this message when I tried to buy the book: "Are you sure you want to buy Wine for Dummies? Books can only be read using iBooks on a compatible iPhone, iPad or iPod touch." To complete the test, I bought the book and opened the EPUB file downloaded by iTunes with Adobe Digital Editions. Essentially I got the same results as you describe. i.e. the book is not readable in ADE.
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