I've heard shoplifters make the same defense for stealing a 25 cent package of gum. A thief is a thief. He usually steals because the opportunity is there to steal and the temptation to instantly gratify his impulsive wants overpowers his flaccid self-control and weak moral compunctions against theft.
It usually isn't any more complicated than that.
Dopers who steal do so because they are compulsive, lack self-control, and demand instant gratification. It has virtually nothing to do with cost. These are not rational, clear-thinking people. If they were rational and clear-thinking, they wouldn't be using dope.
Wow. And from what bathroom stall did you get that piece of wisdom?
Listen carefully. This is a crash course in libertarian economics:
1. Man gets legal drug.
2. Man increases his drug usage.
3. Man gets fired for being too stoned.
4. Man kills and/or steals to get money for his drug habit.
5. Society pays the debt.
If that's too hard for you to follow, then legalization is not your biggest problem.