Why do some Kindle books I check out from OverDrive say "Kindles and free Kindle apps via USB only"?

This happens on all Kindle books purchased from Penguin Group publishers. Penguin Group does not allow their Kindle books to be delivered via a wireless connection from one device to another. You must first download the Kindle book to your computer and then you can transfer to a device using a USB cable.

NOTE: Interestingly, if you download the book to the Kindle app on your computer, you can then transfer the eBook via wireless connection to any Wi-Fi capable device registered to your Amazon account. You can also download the book directly onto a mobile device if you have the Kindle app. After you have checked out your book and selected Get for Kindle, choose Transfer via Computer and the book will be downloaded to your Kindle app on your device.

*This information was found on the Grand County Library District website.