I do a lot of bird hunting and I can tell you one of the main reasons shooting at birds on the ground is so bad .. is because of the safety of our dogs. Most bird hunting is done with dogs and shooting at running or birds holding cover would be seriously dangerous to the dogs. If you aren't hunting with dogs, you are usually kicking brush with a buddy and shooting into brush with a buddy around could leave you looking for a new hunting partner. Shot quickly loses energy when shot into the air .. and I can attest it doesn't hurt on the way down.

As far as waterfowling shooting birds on the water is also very dangerous .. especially on small ponds and marsh where your shot can skip and carry on towards other hunters and blinds, or potentially hit a dog on a retrieve.

I am sure a portion of it is the "sport" of wing shooting and the challenge of hitting a flying bird, but the safety of our dogs and buddys is by far the main reason.