Why should air vents not be installed on the return piping side

Actually air vents can be installed on a return air duct and at least one large one is needed for proper furnace operation. Vents can mean different things depending upon how the wording is used. Usually a vent is called a discharge air vent when installed on the supply but a intake air vent can be installed on the return also. Usually if many return vents are used a central air filter is installed at the furnace to be sure all dirty air is filtered before entering the furnace.