'Gridiron' came about around the same time as rugby I think; as a result of when they first started the whole 'no hands' rule in football (that wasn't always the case). However, the strict namby pamby European football rules weren't popular over here, so I guess football as a name for gridiron just made sense. Most of our sports in North America don't allow for people falling on the ground whinging whenever anyone gets within a 'foot' of us either
Sorry I just couldn't resist. I used to live next to Arsenal stadium and had to argue for hours about how CFL was more interesting.