I've never done this myself, honest, but I can understand that it is tempting for a whole reason. And I like both ends of the spectrum of justification:

The authors further proposed that retailers bore some blame for the problem. In their zeal to cut labor costs, the study said, supermarkets could be seen as having created “a crime-generating environment” that promotes profit “above social responsibility.”

... and ...

“Anyone who pays for more than half of their stuff in self checkout is a total moron,” reads one of the more militant comments in a Reddit discussion on the subject. “There is NO MORAL ISSUE with stealing from a store that forces you to use self checkout, period. THEY ARE CHARGING YOU TO WORK AT THEIR STORE.”