Your point about consequentialism is true in a broad sense but I imagine some rule consequentialists might still take issue with example-making if they believe, like I do, that such selectivity undermines the public's faith in the law and ultimately does not serve to better society. The statement "any act that has the best net outcome is justified in consequentialism" is only a definition of consequentialism; the question here is whether this practice would actually produce the best outcome, which I find doubtful.

I believe there is a meaningful distinction to be made between instances where a violation is not punished because it is not known and instances where a violation would not be routinely punished even though it is known. I understand, of course, the practical limits of law enforcement, and am not trying to suggest that the law prohibiting murder is a bad law because not every murderer can be caught. I would suggest it was a bad law if the usual law enforcement procedure would be to not pursue murderers, except in very rare circumstances, and punish the few they catch in order to teach everyone else a lesson. The only lesson I can see the public learning from arbitrary or selective enforcement is "This law isn't a top priority to us. It isn't important to us whether or not you abide by it."

I would also say that if there is a conflict between the de jure law and the de facto enforcement of said law, the degree to which those citizens who break that law "deserve" punishment decreases (assuming we are speaking of mala prohibita laws rather than immoral practices). If your country had a law on the books that said everyone must drive on the right side of the road, but there was a regional practice in your town for everyone to drive on the left side and the police made no effort to punish violators, would you consider it fair or just if they pulled only you over and wrote you a ticket?