Why does Big Ma go to Strawberry? Why doesn't she park her wagon near the entrance?

Before dawn on a Saturday, Cassie and Stacey depart with Big Ma in the wagon for the market in Strawberry where they will sell butter, milk, and eggs. At first, she doesn't understand why Big Ma parks the wagon so far away from the entrance to the market, but Big Ma assures her that her regular customers will look for her and then tells her that the wagons near the entrance belong to white people.