Further, in my particular case, which is far from unique, I bought a second PHB because one was just not enough. Fatal with D&D slapped on it would not have gotten me to do that.
Fatal with D&D pasted on the cover, after sales like this, would have lots of 4e books in the used book stores, collecting dust. It hasn't, because, I'll bet, that many of those books are being used.
I'm not sure I understand where your coming from saying that Fatal with D&D on it would even be a vital product, not to mention the top selling RPG.