The “under penalty of perjury” was put in there to sell the American people and interested illegal aliens that FISA was necessary to prevent another 911 - that the government with its secret courts and secret agency actions never to see the light of day would protect us ... it was going to allow our protectors to get the terrorists before they could fly another jet into our buildings. ROFLMAO.
Now these same people want to take away your 1st, 2nd and a few other Amendment rights to protect us even more!
The underlying event was bombing of a CIA office. The fact that the government had conducted surveillance without a warrant complicated the government's case.
Within this opinion are a few items worth noting. Two in particular:
FISA creates a crime for activity SCOTUS views as a constitutional prerogative of the president. There is a good argument that the statute is unconstitutional because it criminalizes constitutionally-protected conduct.
Application for any federal search warrant, not just FISA search warrant, is under penalty of perjury.
https://www.justice.gov/opa/press-release/file/956521/download