There are many issues to be found in Trump's pardoning of Arpaio.  Perhaps the most notable was touched on in the article with:
"such a move "would express presidential contempt for the Constitution."  Arpaio didn't just violate a law passed by Congress... His actions defied the Constitution itself... 
The article also touched on the issue of timing noting that such controversial pardons come at the end of a Presidents time in office but didn't touch on a separate but important aspect of timing.  This is in regards to the appeals process which Arpaio filed with the Supreme Court and was still waiting to hear if the Court would take it up or not.
By refusing to wait until the Supreme Court decided to even take up the appeal only further supports the belief that Trump has nothing but contempt for the Constitution.