The concept of "standing" basically requires that you directly suffer some sort of harm from the act or law in contention; the exact details depend on jurisdiction (federal or particular state in the US). American courts generally do not hear cases in abstract, e.g. "I could be harmed by this in the future" or "I know a guy who was hurt by this" or "someone else could be harmed by it eventually". They require some level of concrete, discernible (and judiciable) harm that can be pointed to. As such, if a claim alleges harm from a law, then the fact that the law has not been, and will not be, used against them means there is in fact no harm for the court to consider.
That said, it is typically the case that the reasonable perception of a reasonable threat from an enforceable law is adequate to constitute standing. A lot of the legal challenges you hear about in the news right now frequently use this, as people challenge the Trump administration's actions on a variety of issues almost as soon as they are officially created. The distinction here is that there is no credible, current threat to these plaintiffs because the law is not being enforced in a way that could affect the plaintiffs. If the policy on how to enforce the law is changed, that situation may change and the plaintiffs (or a new one) may be able to make a new claim.
The ability for district attorneys and law enforcement in general to decide whether or not to enforce a law falls under the concept of prosecutorial discretion. Attorney generals have broad powers and responsibilities, and are expected to administer a just and effective application of their prosecution abilities. The US courts have routinely recognized an intrinsic authority for them to exercise discretion: they are not unilaterally and unequivocally bound to pursue all conceivable charges at all times.
On a practical level, there are issues of finite amounts of money and resources. The state and attorney general's office does not have the manpower and resources to pursue every conceivable case in every conceivable instance. It therefore must, of necessity, make judgement calls on which cases are worth pursuing and which are not.
Additionally, the mere action of pressing charges can have a substantial affect on the life of the defendant regardless of the ultimate outcome. As a (somewhat extreme) example, how many people do you think witnessed the acquittal of OJ Simpson and just decided that, yep, he's innocent, so time to treat him exactly the same as we did before this whole saga? A prosecutor has the discretion to not press charges for reasons including a low likelihood of conviction, and disproportionate impact on the accused.
And the judgment call on the likelihood of conviction will take into account the appeals process. So if the prosecutor suspects that a particular application of law, while permissible by the plain language of that law, would have a substantial chance of being overturned on appeal, such as due to constitutional issues, then they may also exercise their discretion to not proceed with charges. And this is what's going on here: prosecutorial discretion is being exercised to decide which cases of potential violations of the law might be worth pursuing and those which are not. Going by your wikipedia link, the financial limitations were a principle factor in this particular choice of prosecution policy. A reason doesn't necessarily have to be offered in general, as long as it doesn't run afoul of other rights (deciding not to prosecute any white people for a given crime would violate the 14th amendment, for example). In other situations an attorney general may conclude that a successful defense based on constitutional grounds is too likely in certain situations to justify an attempted prosecution.