AFAIK, the FBI can't prosecute US citizens for thought crimes.
But how is a website or BBS showing material NOT a thought crime? You might say; the materials are prohibited. But we could outlaw bibles, and then everyone with a bible would be an outlaw. What ABOUT the bible is illegal? Reading the words, of course. They'll say it's possession, but really, it's in what you might learn, think and how it might change your behavior. No clear smoking gun on Pedophilia.
So this is a thought crime. They can see, view and hear but don't DO. Crime is an act that harms people. Until someone actually affects a person or property -- no crime. The only crime is based on prohibited material.
Is the crime in viewing an actual minor, or in viewing someone who LOOKS like a minor -- or a cartoon? What if I'm married to a 26 year old woman who 4 feet tall and looks really cute? Do I go to jail? Sure these people may clearly be looking for kids -- or maybe someone likes tiny women, but how do you define such a thing and does it really matter?
The user in this case is assuming there is privacy. They are viewing material to get stimulated. They didn't touch anyone.
I hate taking the side of Pedos -- but we don't even know if all these people are actual pedophiles. Some of them might just be into extremes and next week they'll be looking at chubby chicks. Some of them may have been abused in the past. If you criminalize this -- you don't have a situation where people can seek help. There are so many cases in our own history where stigmatizing causes MORE of the thing we are trying to reduce.
This is thought crime -- pure and simple. And if the rights of people who have done NO HARM are not considered, as reprehensible as they are, then the long arm of the law might do a reach-around into something else, like colluding with each other to change laws we think are wrong.