How did Joseph Smith cast out devils?

B. H. Roberts records the first miracle by Joseph Smith:

In the month of April, 1830, Joseph Smith was visiting at the house of a Mr. Joseph Knight, at Colesville, Broome County, New York. This gentleman had rendered the prophet some timely assistance while translating the Book of Mormon, and he was anxious that Mr. Knight and his family should receive the truth. While in Mr. Knight’s neighborhood the prophet held a number of meetings. Among those who attended regularly was Newel Knight, son of Joseph Knight. He and the prophet had many serious conversations on the subject of man’s salvation. In the meetings held the people prayed much, and in one of the aforesaid conversations with the prophet, Newel Knight promised that he would pray publicly. When the time came, however, his heart failed him, and he refused, saying that he would wait until he got into the woods by himself. The next morning when he attempted to pray in the woods, he was over-whelmed with a sense of having neglected his duty the evening before, in not praying in the presence of others. He began to feel uneasy and continued to grow worse both in body and mind, until upon reaching home his appearance was such as to alarm his wife. He sent for the prophet, who, when he came found Newel in a sad condition and suffering greatly. His visage and limbs were distorted and twisted in every shape imaginable. At last he was caught up off the floor and tossed about most fearfully. The neighbors hearing of his condition came running in. After he had suffered for a time the prophet succeeded in getting him by the hand, when Newel immediately spoke to him, saying he knew he was possessed of the devil, and that the prophet had power to cast him out. “If you know I can, it shall be, done,” replied the prophet; and then almost unconsciously he rebuked Satan and commanded him to depart from the man. Immediately Newel’s contortions stopped, and he spoke out and said he saw the devil leave him and vanish from sight.
– B.H. Roberts – The Testimony of Miracles – The Evidence of Fulfilled Promises

Why did Newel Knight defend this story even in court?

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