The WSJ editors know that Dems will likely win in the fall and that it could be a multi-term Siberia for their Opinion page (as in "Tell it to my hand").  

They want people to continue to read not only their paper's generally fine reporting but the wrong headed opinions they spew out with great regularity.  Putting a librul on the page makes sense to get eyeballs. If they don't have someone in house to argue with and stimulate letters and readership, no attention will be paid at all...