Why does Ma go from house to house in their community urging people to stop shopping at the Wallace store?

On the way home, Mama stops at different farms and talks to various families about finding another store to patronize without directly mentioning what the Wallaces have done to the Berrys. Moe's father, Mr. Turner, says that he would like to participate in a boycott, but that he has credit at the store and that driving to Vicksburg overnight, like the Logans do, would take too long. Finally, Mama convinces him to promise that if she could get him credit and buy the things he needs in Vicksburg for him, then he would be able to stop shopping at the Wallace store.