The argument is that they already served their sentence. They are free to do as they will. This is effectively in addition to their sentence, retroactively changing the laws. Imagine if suddenly there was a law that said you were to have your tongue cut out if you lied, and that it applied to lies told in the past. How would you feel about that?

The point is, punishments can only be applied to sentences pronounced from this point on, not ones that have already been served or are in the process of being served.

This argument seems based much more in Article 1, Section 9, Clause 3 of the original text of the constitution. Quoth: "No bill of attainder or ex post facto Law shall be passed."

Ok, this part makes perfect sense: it's clearly unjust to add penalties to sentences after the sentencing phase of the trial is complete.

I'm just wondering if there's an argument here past retroactive sentencing.

There are two objections. One based on the article one prohibition of ex post facto laws, and another based on a 4th amendment prohibition against unreasonable search and seizure.

The problem here is the sentence was already handed down by the judge, and the law seeks to retroactively add to it. If a judge handed down a sentence that included the monitoring as part of the sentence it probably passes constitutional muster.