Who Really Kidnapped The Boy?

Riddle: Beckett called the police and said that his little brother, Cody, had been kidnapped. The man told the officers, "I was walking in the park with Cody. Suddenly, a stranger ran up to us and snatched my brother! I told the man to put him down or else I'd call the police, but then he ran away!". The officers asked Beckett to give a description of the kidnapper. Beckett said, "The stranger was tall and thin, he had dark hair, and he was wearing a blue v-neck T-shirt,". After these words, Beckett was arrested for fraud. Why?