Here's a purely descriptive explanation of how our legal system deals with moral luck, informed by research by Kahneman, Schkade, and Sunstein (pdf): people's desired punishment depends on how angry/outraged/indignant they feel about a crime, and people feel more outraged when things actually turned out badly (e.g., when someone dies), so more serious sentences are widely seen as more appropriate when things turn out badly.

This could also be a partial justification of the law's treatment of moral luck: one purpose of the law is to express the people's moral values, so it should (to some extent) reflect their feelings about the wrongness of various actions and how much punishment is deserved by the perpetrators.