why does Joe say he gets mad at the townspeople ?

One day, Matt Bonner's mule disappears. When the mule finally shows up in the center of town, the men who sit on the porch begin to torture it. First, Lum tackles the mule, then five or six other men begin to torment it. Janie feels bad for the mule; she wants to help it but doesn't want to get in trouble with Joe for speaking out. Joe hears her muttering words of sorrow under her breath and decides to do a noble thing. He pays five dollars for the ownership of the mule so that he can protect it from any further damage.