Why are 31.8% of retracted papers not noted as retracted? I read on STEEN, RG. Retractions in the scientific literature: is the incidence of research fraud increasing?. Journal Of Medical Ethics. England, 37, 4, 249-253, Apr. 2011. ISSN: 1473-4257. that:


  Journals do not do a careful job of alerting the naïve reader to a
  retraction notice (table 2). The most common way to alert readers
  about a retraction is with a watermark on the pdf (41.1% of retracted
  papers). A total of 149 papers were given such a watermark and noted
  as retracted at the website as well. Among 305 watermarked papers,
  48.9% were also retracted at the website; of 248 papers retracted at the website,
  60.1% were also watermarked. However, 31.8% of retracted papers were not noted as retracted in any way at all so the naïve reader would not
  be alerted to the fact that retraction had happened.
  
  


How comes 31.8% of retracted papers are not noted as retracted?
The only reasonable answer I can think of is "laziness".

In particular, when journals existed only in print, the only thing a journal could do when it retracted an article was to print a notice in a new issue.  There was no way to modify or recall issues that had already been sent to subscribers.  Some journals may still have retraction policies from that era, which don't contemplate trying to actually do anything to the original article.

Note that the article you quote is almost four years old, so matters may have improved in the meantime.