I don't think Leach's tweet was racist, nor have I seen anyone say that Leach is racist for tweeting it. That's a straw man.
What people have said, however, is that the tweet was racially insensitive (which is not the same thing). Why might some people think that? Instead of immediately lashing out at the "PC police," let's consider it. The joke is that the wife is knitting a "scarf" that is actually a noose -- i.e., her husband is going to be hanged. Why might a black person think a joke about a hanging isn't very funny? It could be, perhaps, that more than 4,000 black people were lynched (or hanged) in the American South between the end of the Civil War and the 1950s. The last lynching was in 1981.
So it could be, perhaps, that we have to consider the context and experiences through which different people with different backgrounds view different symbols. To a black Southerner whose family or community may have been directly impacted by racially motivated hangings just one or two generations ago, a noose is not a symbol that inspires many laughs. If you don't come at it with that context, it can be very easy to say "I don't get the big deal. The meme explains itself." Rather than criticize people for seeing things differently, wouldn't it be better to stop and ask yourself, "Can I better understand why this person feels the way that he does?"
Just my two cents.