The reasoning was that he denied everything as a service to protect the country. From what doesn't really have to be defined. He denied it all to protect the country from a scandal. Boom. Lies told as President in service the the country are apparently protected.

Once he repeated the denials after the trial he blew his cover. He wasn't lying to protect the country now so he just not have been doing it for the country before either, so immunity was revoked.

It's neat to check out, at least once. I took my girlfriend's when they came to visit a couple years ago and we had fun taking goofy photos in the reflection.