Soccer is treated as a very dirty word. Many fans go berzerk and insist that you call it football. In another forum I used to frequent a person used the word soccer and you just had a whole bunch of posters saying that he shouldn't expect people to take him seriously if he uses soccer instead of football. That is prime gatekeeping for me; the view that you aren't a true fan if you say soccer. I also think it's a deliberate attempt to gatekeep American fans.
