How do Tom and Huck treat Muff Potter after he has been put in jail? why did they treat him that way ?

In Tom Sawyer, we see a whole town willing to condemn Muff Potter without so much as a trial. Even before Muff Potter has admitted to the crime, the citizens of St. Petersburg have already charged him with the crime, shouting, "It's him! It's him!" The same man whom Huck and Tom remember as a kind heart who drank too much for his own good becomes a beast in the eyes of the "good citizens." Instead of playing with the other children, Tom would sneak away from the schoolyard to the jail cell where Muff Potter was held prisoner, smuggling small tokens and gifts through the barred cell window ­ an outlet to ease Tom's guilty conscience. Tom and Huck are shown to be more empathetic than the rest of the town.

Tom and Huck knew that Dr. Robinson was killed by Injun Joe not by Muff Potter. So they sympathised with him. Tom and Huck often gave some tobacco and matches to him from the window in potter's cell.