Their existence only proves they haven't been sued yet. But, you have a good point. Perhaps I am mixing Trademark, and Right of Publicity, with Copyright. All of them apply in this case, and have been used in the past to prevent the unauthorized use of MLK's image and works.
As a public figure, MLK ('s estate) has greatly reduced right of publicity, so I'm not sure how you think that would come into this. You are allowed to criticize celebrities. Even in California, MLK's personality rights wouldn't survive his death anyway.
As for Trademark... what is the trademark here, and how would anyone be confused by an MLK-Hitler poster? I don't think anyone is going to buy an MLK-Hitler poster thinking that they are getting MLK-brand doughnuts (or even MLK-Hitler brand doughnuts).
