I did some more digging. It doesn't address the issue of fan fiction and copyright infringement in general, but with respect to these books in particular, JK Rowling sued and then later removed the lawsuit and said that she "supported the novel and any others like it."
http://en.wikipedia.org/wiki/Legal_d...Norman_Lippert
I understand that this site has a complicated task of abiding by copyright in many countries, but in the US at least, this series are very much legal.
eP