I was thinking about this because I'm fixing lights at a few Burger Kings today and have noticed many people ordering the Big Fish meal (which I also had). I don't celebrate Lent, I just really like the fish sandwiches.
I have heard many rumors that the no meat thing is a completely man-made rule by the Vatican because of poor fish sales. I don't really believe that because if you look at Italy you can kinda tell that seafood is a big thing there, much more than beef.
I haven't found a solid answer on that yet.