What I don't understand about Snowden and PRISM

I value my own privacy, and I understand the importance of government secrecy. We can argue about what should be private or secret in another blog post. If maintaining the secrecy of the PRISM program was so important, why not just lie about it? If I were Obama, I'd brush this off by saying it's another conspiracy theory. I'd say something sarcastic like "So this guy has evidence of a secret spying program? Does he also have my real birth certificate?" The White House press core would laugh, and the nation would go back to talking about Honey Boo Boo and Justin Bieber. But by prosecuting Snowden, the government is admitting that PRISM exists. Why is putting this man in jail more important than protecting the secrecy of the program?