I think something else that might be at play for us that think this isn't appropriate is our own understanding of two things - privilege and signalling. I'm no expert in these sociological areas, but this is my understanding. I may be wrong, and am more than willing to hear other opinions, but here's the short version in my mind: these white kids are able to, from a place a white privilege, indulge in stereotypical black foods, which have been used to place a negative stigma on black people. When they go home, the return to their white world. They are also signalling that "this is a funny thing to do" (because of these negative stigma), and that are allowing themselves to descend towards what is considered (by those with privilege, through the stigma) a lower, more base state of society. It's not the foods themselves, but the combination that creates this atmosphere.
A longer explanation: a lot of people have a knee-jerk reaction from arguments about privilege, partly because it is used by people who are flexing their lefty muscles for the first time, or don't quite have a good grasp on the arguments they're trying to make, but stick with me. The idea is this: that we are defined by characteristics, some we have control over, and some we don't. Some of these characteristics are visually apparent, and some are not. These characteristics make up you.
Such has:
What is your sex? Male, Female, other
What is your gender identity? I identify as - male, female, neutral, etc.
What is your race? white, black, hispanic, asian, etc.
What is your sexual identity? Straight, gay, bisexual, asexual, etc.
What is your height?
What is your weight?
What is your age?
What is your income?
etc.
As a society, because of the cultural baggage that we carry, many of these characteristics create levels of privilege in society. For example, a white person may have an easier time getting a particular job, because the person hiring might hold a set of negative feelings about other races. This can be from an actual negative bias, or just a fear of otherness that doesn't come from a deliberately bad-meaning place, but still plays into cultural biases. A woman might have a tougher time getting a job, because the employer is worried that she might get pregnant and leave, or doesn't believe a woman can do the job.
At any rate, a lot of the characteristics that make up us are visual and cannot be removed. This is what creates the unsavory aspect of this party, when the white kids are home, they're still white. They benefit from the social privilege that comes along with that characteristic. The symbols and negative stigma from these foods still remain for the black people of society after the party is done. A black person on the street still conjures up these images in a bigot's mind after the party is over.
As for the signalling argument (I'm slightly worried that I'm not using the right word here, perhaps dramaturgy is a better word, but I think it will make sense), what I mean is that when these white kids throw this party, they are playing the role of black people as a comical display. A black person person's level of privilege does not allow her to play the role of a white person comically in the same way. "Acting white" is often actually labeled as a positive thing by people with higher levels of privilege, because this is seen as the "proper" way to act.
This comes into play when we talk about humor a lot, I think. It's possible for individuals like Dave Chappelle to produce the humor he does because he is coming from a black man's level of privilege and attacking the stereotypes that are inherited from his level of privilege. It is not as funny or acceptable to me when Jeff Dunham has his jalapeno on a stick, because he is simple a person of privilege attacking stereotypes from that place of privilege. It signals that he views these puppet's characteristics as beneath him.
I think the point that people that view this party as wrong are trying to enforce is that playing these roles enhances white privilege, and keeps the structures of privilege in place.
I am sure that a lot of people will find a lot wrong with these arguments, and I am willing to shift my views based on the input from others. This is my own I Took a Sociology Course One Time level of understanding, though.