They are afraid to print controversy. If people want to boycott a paper because of some of its content, then that paper is doing a good job. It is reporting the news. Comics are more than just another form of entertainment, they are also an avenue of expression. Art is an expression of what one feels or believes.
Comic strips are a form of art.
Newspapers also contain political commentary and views of individuals. Commentaries and columns are individual views of a situation or the world. I do not see the difference between that and a comic strip. Hart's strip is not that bad.