Mr. Soltani also claims that it is somehow improper to use
  any technology other than browser cookies to track website
  activity. In fact, countless online companies, including
  other major analytics providers, use a variety of different
  technologies to provide these services, including the
  persistent technologies Mr. Soltani targets in his paper. 

That's not a justification for doing it too. It's merely a lame excuse for unethical behavior.

Also, I don't care if the guys at KISSmetrics are nice, kitten-loving jolly good fellows - that has absolutely zero relevance to the case at hand.