It actually doesn't, which is why this is an INCREDIBLY stupid move by the FBI.

Evidence submitted that was recovered by a private citizen of their own accord is admissible in court even if there was no search warrant issued or other illegal activity occurred to gain said evidence (breaking into somebodies house and finding CP on their computer doesn't give grounds for dismissal of the case, although the person who did the B&E could still be charged for said act). This is the private search doctrine, and it has well established case law.

However, just because you are not on government payroll you are not free from being considered a government agent. If a government employee actively encourages you or incentivizes you to perform a search you can (and likely will) be considered an agent of the government when evidence is submitted at trial - meaning the 4th amendment can be used to throw the case out since no search warrant was issued.

This case gives anybody who has been convicted and had evidence submitted by a Geek Squad employee grounds to appeal their case, or for active cases the charges dismissed. What was the FBI thinking?