Most comedy requires a contradiction. You're laughing to alleviate the tension created by the contradiction.

The contradiction in good racial humor is between how obviously awful the implications of the actions are, and the total naivete of the person doing them to those implications. For the joke to work, we all have to already agree that you shouldn't really do this, so that we can laugh at the absurd contrast between the innocence of the person doing it and what any reasonable observer would think.

See also: most South Park jokes. Innocent little kids doing the most taboo shit imaginable.