It is not worth the time, effort or money to nab a person who has only downloaded a picture or two. The best they could get is a charge of "viewing" and that is iffy at best.
They got the ones they wanted, the ones that were preying on children, taking pictures of them, luring them in, molesting them, etc.
Of the other 6900, probably a good 90-95% were scared off of any sites they register to just by the sting being made public. That alone was all the FBI probably was hoping for in those cases.
I disagree. A single visit would likely be insufficient to support a conviction. But repeated visits would likely be more than sufficient. These more superficial and passive visits need not be prosecuted as felonies, and could be handled as misdemeanors. The point is, ream out 1,000 or so of the dedicated peekers, slap them with misdemeanor convictions and you accomplish two things: 1) you build a record against them should they not get the message and persist in their activities, and 2) you increase the fear factor exponentially and scare off vastly greater numbers of potential peekers as word leaks out into society.
When Guiliani cleaned up crime in NYC he did it by directing his police officers to concentrate on many minor crimes and offenders that had been largely ignored by his predecessors. Nipping relatively minor misbehavior in the bud early is key to preventing much larger problems and crime waves later on.