This was posted to the Smithsonian Institution's website last year as part of an article.
The article is still there, but it no longer references the image.
Fortunately I saved the image.
The academic reason you seek is that it isn't ok to mock the oppressed.
It is only ok to mock the oppressor.
If you are white, you belong to a race that no matter what you do it means you are an oppressor.
It is not ok to make fun of some one that isn't white because anyone that isn't white is oppressed by the existance of those that are white.
It is ok to make fun of someone that is white becaues existing as a white person causes oppression to those that are not white.
There is a consistant logic behind it.
I hope it answers your question.
The only solution that will work when this logic is applied will be to end white people.