You don't live in reality because you dismiss the possibility that someone sees the link and does indeed spread it around outside assumed child porn communities. That it might not have happened in this case does not at all mean that it can't and won't happen in the future. BTW, even if you don't additionally obfuscate the URI you can draw other assumptions about the content if you see 4yo_suck. Not everyone has child porn on their minds all the time plus it sort of needs some knowledge of English to recognize it as such.
I live in reality because I understand that while that's a possibility there are a good number of facts that rule it beyond probability. This link was visited by the particular person on the very day it was posted, which lessens the chances of dissemination by other people. It's also HIGHLY skeptical that someone would send their friend child pornography as a joke. It is explicitly labeled, and since that this is a Doctoral resident that teaches college History classes in the U.S. it's a pretty safe be that he speaks English.
Of course, that alone wouldn't be enough to convict the man, but it's enough to send out the FBI to search. If they had got there and NOT found him destroying hard drives that still managed to have a few files of underage girls on them, he wouldn't have been convicted.
The reality of the situation is that the circumstances surrounding the search warrant are very clear and it makes absolute sense they would investigate it. Should someone have been the victim of a prank in the future, or whatever, that can be dealt with on its own, hence why we have a whole legal system.
~Jason