Let's say that Joe hates John. Joe knows that John is security-aware and has encrypted all of his hard disks and storage devices. John keeps the decryption phrase in his head.
Joe makes an anonymous telephone call or otherwise reports John to the police, claiming that he knows for sure that John has child porn.
Will the cops now go home to John and demand that he gives the cops all of his hardware and tells them the decryption phrase for them to go through it in the headquarters? My skin starts crawling at the thought of my data being looked through by anyone, including cops, who might be making copies and then leak them due to malice or incompetence.
Even if they find nothing illegal, John will feel extremely violated after that experience, and must assume that all of his data has been copied/stolen/compromised. He initially refused to tell them the decryption phrase, but they threaten to lock him up indefinitely if he did not. They may also have turned off their bodycams and smacked him around a bit.
And even if he consistently refuses to tell them the decryption phrase, and they are unable to brute-force it (who knows what kind of hardware they have access to these days?), they still may have taken copies of the encrypted data for "future brute-force attempts", so John will always wonder if his data is compromised or not, even though he didn't even actually have any child porn or anything else that's illegal.
Is this really how things are done? I have a feeling that it's exactly how it happens, as long as the reporter is convincing/serious-sounding.