why do tom and huck take an oath in the graveyard?

In The Adventures of Tom Sawyer, Huck Finn tells Tom the cure for curing warts and that is what takes them to the graveyard. Hucks states, ""Devil follow corpse, cat follow devil, warts follow cat, I'm done with ye!"(Twain).

The catch is that Tom and Huck have to swing a dead cat over a freshly buried dead man said to be evil and state the above encantation. So in the light of a full moon, Tom and Huck go to the graveyard and see Injun Joe murder Doc Robinson while graverobbing. Muff Potter, the town drunk is implicated, but fear and a secret oath keep the boys from revealing the truth.

Why couldn't Trim sleeo?