Life sentence for $20 worth of weed is certainly questionable. However you'd expect someone who's been convicted 3 times already to have come to terms with the shortcomings of his/her particular lifestyle. I'd imagine that after serving his first sentence, he would've already decided that going back to prison is an acceptable risk when it comes to fulfilling his needs.
Don't do the crime, if you can't do the time. I understand that the guy might have been dealt a particularly shitty hand from the start but that's no excuse for not taking full responsibility for one's actions/choices.
He's homeless and out of a job, so plenty of time. But it is backwards, he's just a small catch in a much bigger scheme, it does not solve the crime, he will be replaced with yet one of the many stumped down people that got bitten by the economic crisis. He might be better off, food and shelter for the rest of his life for only $20,- of investment, that is a bargain.
The real criminals still go on doing their business. This man serving his time will not make the world a better place. It will not get rid of other small crumb dealers, they should focus on the real crimes behind it and spend the tax money there, not on a life in prison.