Why does Billy set a wolf trap in the vaqueros' smoldering campfire?

Two days later, Billy travels back past the place where the vaqueros shared their coffee with him and finds that the wolf has been digging in the campfire where the men had scraped their plates. Billy makes some new sets but has lost the belief that he's going to catch her. Then he sees the smoke from another campfire. He goes there and finds that the vaqueros have recently left the area. Billy, without taking the time to stand on the calf hide or to hide his scent in any way, sets a trap in the middle of this newer campfire. He writes a message in the dirt, warning the vaqueros of the trap, and goes home.