This isn't a problem that is isolated with the Redskins and/or Fed Ex. Did you see how many Bears fans there were in Dallas? And I am not talking about at the end of the game when the Bears were up 34-10. Before kickoff there was loud audible "Lets go Bears" chants.
This past Sunday in Tampa there was a ton of 'Skins fans at the game.
These stadiums that hold 80,000? There is no way to avoid a significant amount of visiting team fans getting tickets. If you are a fan of the visiting team and your favorite team comes to town, chances are you are willing to pull your resources to throw down a hefty chunk of change to go see them.
I know that whenever the 'Skins come to Oakland or San Fran, or San Diego etc etc....I make it a priority to get myself to the game, even if I have to pay above market price for the experience, because I know it is a rare chance I get to see the team live.
Also, you have a significant portion of "season tickets" owned by brokers and websites which merely sell to whoever has money.
It always looks worse than it really is though. If the Redskins are winning the game, and sticking it to their opponent, the visiting team fans will be sitting on their hands and perhaps leaving early. However, when the 'Skins are stinking it up, you have a situation like you just saw in Dallas, which is the home town fans leaving early, and the visiting team fans moving en mass to the field level seats and giving the impression that they "took over" the stadium, when in fact the reality is it was still probably easy 80-20 home team fans when the game kicked off.