I think we've seen some teams have printouts with them, but I think the editors generally try to avoid that. It does come down to cost and capacity -- with the IPad and the various other tablets and smart phones, it can be difficult to print something out. I remember noticing one team this last leg or two getting directions from a driver who used his GPS (Garmin, TomTom) to get directions for a team and he had taken the viewer off from the windshield to show the team what route to take.
So I don't think they are prohibited, per se, it just that they often already have directions (many teams do ask on their flight, train, bus if they have a destination in a city already). Since this is an activity that often has them interacting with locals, I can't see the reason for not allowing it.