Increasing the number of maps per day/week that someone can obtain lowers the value of the items. They aren't very "treasured" if the volume available increases by that much. And for the players that only harvest the map and sell it, it also drops the value of the map itself.

Technically speaking, a player that really wants to do maps can do as many as they want per day (buy them). Keeping a limit on the number of maps in circulation keeps the value of all things related to it, higher.