Just because they let one kind of "psychic" in doesn't mean they'd let all
psychics in. They likely understand their own psychics, but may not believe or understand the Vulcan mind meld. Besides, what else would Spock say? Of course he'd end up saying "Mr. Scott did not commit the murders", because he's a friend and colleague of Scotty. Why should they believe Spock's claim of Scott's innocence? Just because he touched Scotty's face and winced a bit? How are the Argelians to know that something psychic really happened?