Originally Posted by Nate the great
Do you know what? You could be correct.
I was going to dispute the relationship between Nook and MS DRM, but the NooK HD also supports Ultraviolet DRM. Who knows, that might be the reason you need to have WMP installed.
It will support UV, but doesn't so far (mine says coming soon when I try to enter my account info). Also UV doesn't require WMP, unless this is something UV/B&N specific.
I'd guess as Geert says it's an MTP thing.