Just today I read a news story about a junkie who went out in the street in his neighborhood and started shooting at people. Every bit of evidence suggests he was high as a kite. Luckily he didn't injure anyone, just did some property damage putting bullet holes in cars. He ended his existence by taking shots at the police when they showed up.

Some guy stoned out of his mind on weed may be harmless, but this is not universally true with all drugs and all people. And your idea lacks an explanation for who will pay for all these free drugs. What's in it for me, as a productive member of society, to finance those drug habits?