I would assume it was a reply to a popular comment, and it being a cartoon increases visibility for people clicking through. Also not impossible that some paid

I would assume it was a reply to a popular comment, and it being a cartoon increases visibility for people clicking through. Also not impossible that some paid or highly motivated people would reverse image search the picture in question and try to make problems for the people posting it. I do also think that posting a racist cartoon is grounds for a professor to cut their ties to a PhD student. A PhD student isn't worth that much, and standing by them through accusations of racism is probably not worth the risk.