The Crimson Binome
Hero
The rules in the PHB and DMGs do say something to that effect. They kind of spread the message out, among various places, but it's there.Do the rules say, "The GM must have a fixed map, and populated map key, and that may not change during a session of play?"
The contrary position, that it's okay to change things based on personal bias, is not as present. I'm not sure how anyone could come to the conclusion that it was acceptable or expected, unless they were bringing it in from another game. But there are a few inconsistencies that could be argued, if you really wanted to.