From their FAQ
"The Barnes & Noble eReader is an application used to read Barnes & Noble eBooks on your iPhone, Blackberry, Windows PC or Mac. Without it, you will not be able to read Barnes & Noble eBooks. Our eBooks are encrypted to protect the authors' work. Thus, other eBook Readers will not work with Barnes & Noble eBooks. The Barnes & Noble eReader can display styled text (italicized, underlined, etc.) and formatting as well as perform functions not found on other eBook readers. The Barnes & Noble eReader is FREE and is supported on multiple devices."
Sounds like they developed their own!!?!?!?!