I don't see how they had any other choice than the offer they made. He didn't have 20 years at the time and he did admit to acts that support felony level charges, I don't see how they could wait and still project any justice in the matter.
Again, the punishment does not fit the situation.
Resign and no retirement? I wouldn't have taken it. Would you?
Non-judicial punishment, IMO, would have been the way to go.
It could have been more than an a$$ chewing and discipline would have been maintained.