This was done as a practical joke. The most frequent question I have heard is not about why someone would do this, but how on earth someone could think this was funny.
My generation is in their thirties, and living on the West Coast I have sometimes needed to explain the history and gravity of the watermelon.
I live in a city often labeled as liberal and progressive, yet this city is in the same state as the whitest big city, Portland. As a woman of color, this incident and following response has shaken my confidence in this town and this state, as well as my feeling of safety at work and has sent me in search of employment in more multicultural locations.