We haven't, and I think it is fairly safe to say that rehabilitation and counciling programs to stop shoplifting would be far more expensive and ineffective than imprisonment; so we shouldn't hold out for a perfect solution over an imperfect one.

Also, people don't shoplift just once for fun. A "relatively small loss" is quite large by the tenth time it happens because nobody got punished.

EDIT: For "posting too fast," I would like to address the below comment. For the "it would be cheaper," you should look at how much rehab and counseling costs. For drugs, a 30-day rehab can hit as much as $27K, even if shoplifting rehab (if there was such a thing) would be cheaper. But even if it was cheaper, property crime has a recidivism rate of 78%.

But secondly, you try to rationalize the cost of the stolen item to the cost of imprisonment. This doesn't quite make sense, because A. It values the losses of time, security, customer inconvenience, etc. to the store at $0; B. The store is paying taxes for... what, when they have no guarantee of safety; and C. Apparently the value of justice being served is $0 as well (it's only worth imprisonment if there's lots of money involved - but then, of course, why imprison anyone for their third DUI that didn't involve property damage?).