Isn't that always the way with those who perceive themselves to be associated with the One True Whatever. I believe that logical fallacy is called "No True Scotsman", coined by Anthony Flew.
Here's the Wiki linka nice discussion:
http://en.wikipedia.org/wiki/No_true_Scotsman
I've been dealing with this one a lot lately without having had a convenient way of referring to itit's nice to have an off-the-shelf dissection of the fallacy. Many thanks!