It isn’t?
Any possible plea bargain in this case is a travesty of justice - PERIOD. He is guilty of murder - period, and should be treated like the murderer he is.
Unless he purposely hit the other car he did not commit murder. Manslaughter or criminal vehicular homicide is the usual charge for killing someone while drunk driving. The "plea bargain", at least as far as the charges go, doesn't appear to be any real advantage to the defendant who will most likely be sentenced under sentencing guidelines. Any lesser charges that will be dismissed are usually of no import as the sentences for them usually run concurrently.
I think the prosecutor is probably acting responsibly, the Federal government who allowed this invader into our Country did not, of course, act responsibly.