Well I would imagine it's because people have their own opinions, and put them forward as such rather than looking at things exactly as the manager would.
Just because Koeman wants a target man, doesn't mean a fan can't think that would be better off with a different player up front, same as fans are welcome to think that Martina shouldn't be playing ahead of Kenny, DCL isn't a winger or Sigurdsson would be the biggest waste of money since my old man spent £500 on a microdisk player just as the ipod was released.