SE made a statement at one point in the past about why you can only change plates in city states/the reach. It has something to do with getting killed in a PvE area and that messing with the glamour code. Because you cant "die" in a city state and they are considered safe zones, it just made more sense to limit it to there. Id be interested to see if a BLU can self destruct and then apply a glamour while dead inside a main city.