Black people and Grape soda WHAT?

my friend send me this video on youtube and he said it was funny but racist, but i didn't get any of it!! I said "I don't get it" and he said "You know ... Black people ... Grape soda.. you kno?" I said "no??" he signed and left, the video was Harold and Kumar : Guantanamo Bay Grape Pop Soda Scene , but i didn't get why the bald man was dropping grape soda front of the other guy, HELP??