Power of God vs. power of Satan

Why is it that when great miracles are done in God's name, people don't believe it could happen and say that the person must be possessed of the devil or something like that. How come they think it is possible that someone with the power of the devil could work a sign or miracle, but that someone could not do it with the power of God?

Why is it easier to believe Satan could do it than God could do it? Just wondering...
For example-

Helaman 16:6

But the more part of them did not believe in the words of Samuel; therefore when they saw that they could not hit him with their stones and their arrows, they cried unto their captains, saying: Take this fellow and bind him, for behold he hath a devil; and because of the power of the devil which is in him we cannot hit him with our stones and our arrows; therefore take him and bind him, and away with him.