The idea is that while the investigation is ongoing, of course it's going to be classified. When the subject is finally arrested and taken to court, or when the investigation was over, then the wire-tapping became unclassified as a matter of governmental record (as it should be imo).
What the government has been trying to argue or promote is the idea of a NEVER ENDING investigation, such that from the moment these systems are in place and turned on, subjects are "investigated" probably for the rest of their lives.
This is the TIA/Carnivore system that was hastily put aside due to massive public outrage, but the promoters of this new omnipresent surveillance state simply went back and developed their shaky legal framework to justify it instead.
Do not believe them, none of this is legal, prudent, or necessary. It exists to create an omnipresent surveillance state in which guilt is assumed and most of us will never be truly free.
All this is quite well established in other areas of police-work and prosecution. We have specific, individual warrants with normal judicial oversight for a reason, and broad general warrants giving carte blanche to follow certain rules in collecting data simply shouldn't be allowed, because there's too much scope for abuse.
So I think there is a third way in the middle between making all investigations completely open, and allowing secrecy to consume every operation of government.