Seems like another case of a procesutor wanting to make a name for himself at the expense of sum dumb shmoe.
Partly that, plus a lot of arrogance. Prosecutors often throw the book at someone who won’t “play the game” of plea bargains.
There was an appeal about this in federal court, saying that offering a lesser charge in return for testimony was equivalent to bribery. But it didn’t go far. In my mind, it was right on the money.