Quite simply the use of these ramps when you put them behind a wall is RDR because it is soooo unfair on the raiders. As a raider you have less than a second to figure where they shot you from and shoot them back. There doesn't need to be a specific part in the rule about it as it states "Players who are raiding you must not be facing an unfair advantage." and lets be honest, if you have .5 of a second to react and shoot someone and the other player has all the time in the world it is really unfair.