Even if you are super cynical, throwing a shoplifter in jail accomplishes what? If they have a job, they'll lose their job. If they have a home, they'll probably get evicted. They'll have to go through a court system that has to deal with this. People will have to do a bunch of work to move this person through the system.
And at the end of the day someone goes to jail a bit and leaves, and there's no safety net, so they're just permanently worse off. Congrats, you not only wasted an outsized amount of resources for petty theft, but now the person is back in society with less means of getting anything done in a legal way.
If you consider people to actually be people who you want to see improve, there's a hell of a lot better things to be done than putting people in jail for petty theft. At least in the US system.