If you were poor, you'd be carjacking people?
I agree with the OP - as a society, we should look more at aligning incentives rather than instilling morals.
Another huge area this comes up is the war on drugs - if you're caught with drugs, we slap you with a felony that ensures you can't get a real job... pushing you right back to drugs.
I could say the same thing for any sort of crime. If you're an accountant, and you get put in jail for embezzling, that conviction is going to prevent you from getting another job as an accountant.
While there have been a few controversies about jobs that the law excludes felons from, in a lot of cases there's nothing preventing you from hiring a felony drug criminal. If you personally are fine with drugs and you think that committing the crime doesn't make him a danger to your business, go ahead and hire him. If you won't, it isn't the conviction that's keeping him from being hired, it's the crime; the conviction just lets you know that he committed a crime.
If I'm understanding you correctly, you're arguing that a drug user is less employable (perhaps because you believe drug users are untrustworthy or unreliable), and this is the reason they aren't hired.
But a conviction for a drug crime years ago does not mean someone is a drug user today. It is the conviction, not drug use, keeping them from being hired. A drug test would make more sense if you want to determine whether someone is a current drug user.
And besides, without the conviction you may be unaware of their drug use. If someone has a drug habit, but nobody can tell, what exactly is the problem? There are plenty of "functioning alcoholics" in the workforce.
It has been my anecdotal observation that it is more common for small, local businesses to "look past" prior convictions when hiring and be more willing to take chances on their neighbors.
Large corporations with big HR and legal departments typically have a dimmer view of things however.
Right or wrong, it is harder to get a job with a past conviction. Without a job, it is difficult to earn a living, feed and house yourself and your family. When people are desperate and unable to survive through legal means, they resort to whatever it takes to survival. It's human nature.