Why should it be legal? Sure, that would be the question asked by the authoritarian, because people only have those rights they are given, those rights their dictator suffers them to have. But alas, we threw those shackles off long ago here in America. The question should instead be "Why should it be illegal?" Here, prohibitions are not put in place without good reason, with few exceptions. The most glaring exception is Marijuana. Criminalized for reasons known to be false for ~50 years now, for some reason the federal government has always resisted reforming Marijuana laws, to the point that Obama basically called off enforcement on the consumer level without changing the laws! And never, in all this time, have they given justification for this restriction of personal freedom. Nor have they, until very recently, allowed any unbiased research to be done. Somehow, the question "Why should it be illegal?" was never answered, and yet still it remains illegal. Why should it be legal? Because this is America and there is insufficient reason to restrict this personal freedom, that's why.
What does this accomplish, this "we won't legalize it but we won't enforce the laws" attitude? It encourages a thriving black market. The risk, which was already low, drops lower. No sales tax, licensing, inspections, tax IDs, etc. What fool wouldn't start selling weed in high school? A time in your life when you know the records will be sealed when you turn 18, and the courts are reluctant to send kids to jail over weed. The Marijuana itself is unregulated, subject to questionable growing/drying/storing methods, possibly exposed to various mold or fungi, etc. The strength of the Marijuana you buy is uncertain until you smoke it. This is the harm Prohibition causes: it encourages and lowers the risk of the black market, provides a source of illicit income, and poses a small routine risk to the consumer in the form of unregulated consumables.
The benefits of legalization are real and significant. The market already exists, from growers to distributors to consumers. Money wise, everybody but the black market entrepreneurs benefits. Real, legal jobs open, both in day to day growing operations and selling/installing/improving technology/equipment/real estate/infrastructure. The tax revenue.... One need only look at Colorado and the other legal states to see the benefits there. Some of the money would have to be used on education, regulation, and enforcement expenses, but the government makes out pretty good in the deal. More importantly though, it would provide some much needed credibility. As a teen, DARE was a joke. We listened to them tell us all drugs were equally bad. We tried weed, and it wasn't really that bad. "Were the rest of those drugs not really that bad too? Did they lie about everything? Now I'm curious......" This is the lens that all information received via those channels ends up being seen through, even though they were mostly right regarding every other drug. Most problems stemming from legalization involve people breaking the new laws and growing farm more than they're allowed, and/or transporting Marijuana from legal areas to illegal areas. To this, my response is: Enforcement needs to step up, understand their role in ending prohibition and embracing legalization, and do their job. If I'm allowed 6 plants and I grow 200 to sell on the black market, that's an enforcement problem, not a problem with legalization. And if Marijuana was legal in every state, there would be no illegal areas to transport it to.
One final thought: Colorado banned the sale or production of edibles in the form of childrens candy. Infused gummy bears and such are illegal, and I fully support this. No sane moral adult would want to see a child accidentally get high. Legalization obviously wouldn't mean total deregulation. Driving high, being high on the job, underage use ... These issue will come up, and they are not something we should just accept. But they will happen either way, prohibition has failed to prevent or even reduce these issues, and has done quite a bit of harm along the way.