Why does jay-walking turn you into an evil anti-american, versus walking ten feet down the road to use the crosswalk?
It doesn't, and the law is mostly ignored*, unless your flouting of it is causing problems. For example that guy that's always hanging out near the high school with 10lbs of weed in his pack is probably not a being of sweetness and light, but if he's slimy _enough_ it will be difficult to catch him in the act. Considering the ambivalence and outright dread of legalization among a not-insignificant portion of the population, it is reasonable to give the authorities the power to preemptively intervene in the most problematic potential side effects, forseen and unforseen.
And yeah, I know that was a literal "think of a children" argument there, but I think this is one of the times when that is a legitimate concern. I don't know about you, but I certainly remember such people from my high school days, including cases of kids ending up dead because of (presumed) business disagreements.
* Of course the problem with any law often ignored is that the authorities can also decide to use it to harass people for ...unauthorized reasons. It would be nice if there was a way to explicitly state that certain laws exist are intended to be applied with discretion, and give them additional safeguards against abuse.