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.