TGV has a compulsory reservation with a seat allocated, but this comes with exceptions and Charles de Gaulle to Marne la Vallée the most outstanding of them.
This route is served by cross-country services (Lille / Brussels to Nantes, Lyon...) and basically, SNCF does not want to allocate a seat for a 9 min journey while it would have a much higher value for long distance (CDG-MLV being the bottleneck section in terms of train loading), deciding that for such a short time you'd probably remain on the vestibule with your suitcase.