It's not actually. The barrier crossing the over to the allied building is to prevent easy access to allied spawn positions, which if they could axis would easily reign chaos down upon them. By both 2nd flags there are boxes and what not, and both have other means of entrance. Also access to the buildings through these spots wouldn't further increase spawn kill rates. Plus I wouldn't think it to unreasonable to see WWII soldiers boosting each other to get into better spots. You're talking about guys that made sticky explosives out of socks here.