Selective enforcement is the road to despotism.
What do you mean by that? District Attorneys commonly choose not to prosecute. Its possible that he didn't think he could win the case in court.
Hypothetical scenario: Two politicians do the same illegal act. One is an R, the other a D. The D is allowed to resign, that is considered punishment enough and the matter is dropped (Jim McGreevy, Elliott Spitzer, Bob Torricelli, etc). The R is doing hard time.
What was that about selective enforcement?