Boy this is confusing. If Wisconsin has a law exempting ordinary spankings from misdemeanor assault, it sounds like she pleaded guilty to a noncrime. The judge may accept the facts... but he can't have the law dictated to him by the prosecution.
Although I've requested that we see a copy of the complaint, my guess is that she was charged with a crime that carried a higher penalty (either a C, D or E felony) and it was reduced to the misdemeanor in the plea bargain. Once she plead guilty to that crime, the judge uses the complaint as a basis for her sentencing.
In this situation, the judge probably accepted the recommendation, probably a joint recommendation (from the defense and the prosecution) about sentencing. However, the judge does not have to accept the sentence recommendation and can use the complaint as a basis for sentencing after a guilty plea has been entered, and it does happen if the judge feels the sentencing recommendation is to short or too long. I have seen a few cases in which the defendant pleads guilty, but there is no sentence recommendation. In those situations, each side argues for sentence.
The key factor is that she plead guilty. Had she taken this to trial and been found guilty, the judge uses the evidence, testimony and victim impact statements in his (or her) sentence. Had she been acquitted, she would've walked.