How do black people show their appreciation to Atticus after the trial? Why are they doing this?

In chapter 22, Tom's father sends a chicken over to
Atticus on the morning after the trial. Calpurnia cooks it and Atticus remarks that they
don't even eat chicken for breakfast in the White House. Cal then informs him that
Estelle from the hotel sent over rolls, too. When Atticus looks surprised, she shows him
the kitchen table, which is loaded with "hunks of salt pork, tomatoes, beans, even
scuppernongs," pickled pigs' knuckles, and more on the back steps (213). Calpurnia
translates for Atticus:

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"They—they 'preciate what you did, Mr. Finch.
They—they aren't oversteppin' themselves, are they?"
(213).

Atticus tears up and
says that he is grateful, but they shouldn't do that again because there's a depression
happening in the country and the black community has it harder than anyone
else. 

Tom's family and friends were like most everyone
else during the Great Depression. They may have had some food that they grew or
harvested on their own, but no one had money to pay for legal services. Atticus never
asked for anything, but as Cal said, they appreciated the honest effort that he put
forth for Tom's sake and wanted to show him this in the best way they knew
how.