The DMG also says you are only allowed to use Official Advanced Dungeons and Dragons miniatures when playing the game (on page 11). The infamous "Poker, Chess, and the AD&D Game" Dragon article with it's rant of Thou Shalt Not Change Thine Official Rules (not just for tournament play, the article was explicit that it meant any time the game was played anywhere, if you changed the rules in any way it wasn't D&D and you shouldn't call it that) also cemented the perception in a lot of eyes that the RAW were inviolate and sacred, and that the DM was to interpret the Official Rules, and if the DM violated the Official Rules, he was Wrong (it might be his table, but that didn't change the fact he was Wrong). Sort of like the Judge in a court of law, they have extremely broad authority to interpret the law, but aren't supposed to make up entirely new laws or totally ignore precedent (hope that's not too political).