The logic is that if you have the opportunity to fire a warning shot, the circumstances were not severe enough to justify firing a gun at all. A woman in Florida just got 20 years in prison based in part on that principle: http://jacksonville.com/news/crime/2012-04-21/story/naacp-we...
Note that some evidence suggests she was also the initial aggressor, which likely contributed to her conviction.
Note that some evidence suggests she was also the initial aggressor, which likely contributed to her conviction.