I wouldn’t consider that any part of effectively enforcing the law, simply because a lot of people are arrested and simply let go. It happens all the time. The law isn’t enforced (at least the way I look at it) unless a case is carried through in a court of law and a person is convicted. Then it’s “enforced”.
“Arrest” is a very low bar and just about anyone can be arrested for almost anything. And so it doesn’t mean much. It means something - in terms of “enforcement” if a person is CONVICTED of a crime and there has been “due process” given and you get a guilty verdict. Then you’re talking about enforcement.
I think you make a superb point. The fact is that District Attorneys dismiss cases many times and are selective in what cases to prosecute based on budgetary constraints and likelihood of conviction. Their most effective tool is plea bargaining which obviously the defendant has to agree to and the judge approve.