Deliberately snooping every file on every computer that enters the shop is quite another.
Given the $500 reward, I suspect the latter situation is closer to the case.
Good that they may have caught this fellow -- especially given his profession -- but that's probably rather horrifying for a significant portion of other people who brought their computers in for repair.
Some professions are mandated to report any suspicion of child abuse. In NY, any teacher, medical provider and others can lose their license for failure to report.
Your general obligation, or obligation as an IT guy may vary... and you may have liability risk.
Personally, I'd consider it an ethical duty to report such a thing. But I wouldn't do so in an official capacity without advice of counsel.
... by state.
No.
If you come across child pornography (in the U.S.), you are legally obligated to report it.
That sounds even more like an incentive to plant.
Whoever, having knowledge of the actual commission of a felony cognizable by a court of the United States, conceals and does not as soon as possible make known the same to some judge or other person in civil or military authority under the United States, shall be fined under this title or imprisoned not more than three years, or both.
For anything to do with children, quite a few professions are mandated reporters including LEOs, all public and private school personnel except volunteers, social workers, and several other rather large fields. Depending on the state that list expands considerably. Even the clergy of religious institutions are included in 27 states, superceding confidentiality.
Other than LEOs, mandated reporting for general crimes depends largely on state laws except for fiduciary duties.
And if the FBI is advertising cash rewards for "accidentally finding" illegal content, that sounds a lot like an illegal search.