People know that Norman was _a_ Green Goblin, but it's also public knowledge that there have been a number of people to wear the costume of the Green Goblin. We know Norman has friends in high places who feel that he was set up to take a fall, and we also know he can afford the best lawyers, who can take advantage of what happened the last time a Goblin was on trial, and use the existence of other Goblins to put enough doubt on who exactly was wearing the costume during any given Goblin appearance.