The U.S. is a common law jurisdiction, which (among other things) means that when there is a novel factor involved, usually the courts are first to decide how m

The U.S. is a common law jurisdiction, which (among other things) means that when there is a novel factor involved, usually the courts are first to decide how much it matters. Autopilot was a very novel factor at this time, so the courts need to decide if it matters enough to reduce the criminal penalties. In the absence of an explicit law, that's just how it works. The driver doesn't become responsible -- responsibility becomes an open question. In civil law jurisdictions, this works differently.