Why would selling watermelons in bulk be mocking?

Question

The following passage mentions African-American abolitionist Frederick Douglass, but the bold's figurative, possibly racist meaning eludes me.

_{Source: The Rest Is Noise: Listening to the Twentieth Century (1 ed. 2007; but 2008 Reprint ed. exists, p. 137 Top.}

  That tap on the shoulder, metaphorical or not, Will Marion Cook came to know well when he returned to America. He tried to make his name as a violinist, advertising himself improbably as a "musical phenomenon performing some of the masterpieces upon his violin with one hand." Making little headway, he then formed the William Marion Cook Orchestra, with Frederick Douglass as honorary president. At around the same time, Cook wrote, or began writing, an opera based on Harriet Beecher Stowe's Uncle Toni's Cabin. Most significantly, in 1893, he went to Chicago to participate in the World's Columbian Exposition, a momentous event at which America declared its new status as a world power. In an effort to counteract the stereotypes of black savagery that figured in some of the fair's displays—crowds flocked to watch and hear the African drummers of Dahomey Village—Douglass organized a Colored People's Day, which aimed to affirm the nobility of the black American experience. Newspapers mocked Douglass by speculating that watermelons would be sold in bulk. [Bold mine]

Answer 1

This is one of those stereotypes which, as this question demonstrates, is older then we realize. The entry in wikipedia touches on the assumed meaning:

that African Americans have an unusually great appetite for watermelons

but its history seems best summed up in this article from The Atlantic:

The trope came into full force when slaves won their emancipation during the Civil War. Free black people grew, ate, and sold watermelons, and in doing so made the fruit a symbol of their freedom. Southern whites, threatened by blacks’ newfound freedom, responded by making the fruit a symbol of black people’s perceived uncleanliness, laziness, childishness, and unwanted public presence. This racist trope then exploded in American popular culture, becoming so pervasive that its historical origin became obscure.

So to answer the question, the newspapers of the time were ridiculing all the organized events by insinuating that the larger population of attending African Americans would require a large number of watermelons to feed.

Answer 2

Douglass organized a Colored People's Day, which aimed to affirm the nobility of the black American experience. Newspapers mocked Douglass by speculating that watermelons would be sold in bulk.

I think the intended meaning is obvious enough to discern; they were mocking Frederick Douglass's attempt to 'affirm the nobility of the black American experience' by suggesting all black people were worth were as field hands; this is more or less kin with other attempts to mock black people, for example, blacking up as minstrels, golliwogs and what not.