Why doesn't injun joe get punished for body snatching?

In a nutshell, the town had bigger fish to fry: Dr. Robinson's killer was on the loose. Remember InjunJoe framed Muff Potter, and only Tom and Huck knew what really happened, but they were too afraid to come forward becuase they knew first hand what InjuJoe was capable of. The town was too preoccupied with the bigger crime, and eventaully InjunJoe skipped town, so even if they wanted to charge him which most were too afraid to do they weren't able to.