We have to sign off on everything and record how we obtained the information and have been told by a number of lawyers that in no way are we allowed to break the law when collecting evidence that could be forwarded to police or prosecutors.
Private detectives go through the same thing as well. If they are carrying out a private investigation for a corporate client they can't submit evidence that has been obtained illegally to be used at trial. For eg. you can submit evidence from public surveillance, but you can't submit anything that you obtain by hacking email accounts or placing a recording device on private property.
From what I understand, there are very very strict rules about both gathering evidence and then chain of custody. The person who collects the evidence has to sign off on it and then be prepared to testify in a court to back up what they found. I know for certain that this applies in the USA, UK and Australia.
I wonder how the FBI are able to use evidence collected by Anonymous, or if they just use that work as a basis for their own investigations which start from scratch. I can't imagine a judge would be impressed when told that key evidence was obtained via an illegal breakin perpetrated by a group of hackers.
Re: Sabu. I've read everything there is on that case and don't recall a reference to his handlers prompting him on Operation Darknet. The timing also doesn't seem to work - Sabu was taken offline last March while this arrest is the culmination of a 12-month investigation, which would suggest it started around 5 months after Sabu's work with the FBI was completed.
[1] Just a note - I was usually against prosecuting the defacement style hackers or guys who were just poking around for fun. In all my work i've only ever been involved in two cases where evidence I collected (IP addresses, email addresses etc.) ended up being used in an investigation and in both cases it was phishing attacks from Russia.