Well, because it is harmless and not one single person has been injured by it. If someone wants to spend their entire life consuming it, that is their right in a free nation. Just like people who spend their entire life tinkering with old cars, picking up girls or playing video games. Leave them alone.
Plenty of people are literally killing themselves with booze and junk food as we speak but it entirely within their right to do so. Unlike harmless cannabis, booze and obesity have a 100% medically proven record of causing serious body harm.
By its nature, the drug war is dangerous for all citizens in a free nation. It has been consistently used as a mechanism to strip our personal rights. Right now, anyone can plant a tiny package on someone and charge them with a felony. Holding a few grand in cash can result in a seizure until you prove it's NOT drug money. Anyone’s house can be “SWATed” under the guise of a “confidential informant”. This abuse of power happens all the time and goes unpunished: http://www.dailymail.co.uk/news/article-2095772/Cowboy-cop-Jason-Arbeeny-planted-crack-couples-car-seat-escapes-jail.html
I think I said that but:
DON’T GIVE IT TO KIDS!
If you haven't been paying attention, the Left is using SWATting to harass and intimidate conservatives now. They don't even have to allege drug activity. They can claim they heard gun shots and shouting.
Militarized police with no accountability are the problem, not just drug laws.