In the short story The Monkey's Paw why do the Whites react so strongly to major Morris throwing the paw onto the fire?

Eveyone can be tempted by the lure of having their wishes granted, and Mr. White (although skeptical) does not want the paw destroyed before he can see if it works. Morris, having used the paw, says he kept it to possibly sell it. But the implication is that he kept it to remind himself that men should not tamper with powers they cannot control.