It's not just a TREK thing. Some people just don't comprehend appreciating fiction as fiction. Look at the nutcases who think that the Harry Potter books are going to convert children to Satanism or whatever. And, growing up, I don't know how many times I had to explain to concerned teachers and grown-ups that, no, I didn't really believe in vampires and werewolves. I just liked to read about them.
The other explanation, of course, is that the really nutty fans, who take Trek or Star Wars or Twilight too seriously, are the ones who get all the press!