DoD dropped the case after lengthy legal wrangling by Bisonette's lawyers. The Obama administration was taking heat for trying to prosecute a "hero"...and for administration officials themselves leaking a lot of the info about the Bin Laden raid before the book was published. Panetta, the CIA director even wrote a book. So the whole case against Bisonette was worked out in a legal settlement, the result of which is outlined in the OP. His lawyers could point out a number of officials who were publicly "crowing" about the raid so there was potential for embarrassment if the Bisonette prosecution was pursued.
Btw, I find it interesting that Bisonette disputes O'Neil's claim that O'Neil fired the fatal shots into UBL, saying an unnamed 3rd SEAL was the killer. This is almost funny...