Two possible explanations, my own rose-colored version first: I assumed it meant the cowboy didn't "shoot" the steer as instructed, but instead gave it a swift hit with a pan so it would run away into the wilderness and have a shot at life.

The more likely explanation, as explained in a couple editions of "Plains Folk" by Jim Hoy and Tom Isern: "if somebody else's steer happened to come along, then the trail crew could have fresh meat." "[R]ange custom allowed it; besides, if one of your steers got in someone else's herd, you knew that it would be eaten." (as printed in the [Kansas] Council Grove Republican, July 3rd, 1997, p. 4 and August 3rd, 1994, p. 4