In the book In Cold Blood, why did Andy Erhart and Herb Clutter's friends offer to clean the house after the murders?

From the text:

Today this quartet of old hunting companions had once again gathered to make the familiar journey, but in an unfamiliar spirit and armed with odd, non-sportive equipment - mops and pails, scrubbing brushes, and a hamper heaped with rags and strong detergents. They were wearing their oldest clothes. For, feeling it their duty, a Christian task, these men had volunteered to clean certain of the fourteen rooms in the main house at River Valley Farm: rooms in which four members of the Clutter family had been murdered by, as their death certificates declared, "a person or persons unknown."