And you can plan a murder. The conversation itself is not a crime. But it is very damning evidence if the prospective victim that was discussed actually turns up dead, showing that the crime was, in fact, premeditated murder and not a less severely punished type of homicide, and that accomplices were involved. The speech is not the crime. It is evidence of malicious intent if a crime subsequently occurs. It may also be useful intelligence that could allow someone to interfere with a crime in progress.

If you overhear the murder conversation, you might be able to prevent a murder. But those guys could have been talking about their clan strategy for a MMORPG raid, and you simply misunderstood the intent. You don't know for certain until someone acts.

Censorship cannot stop crimes. It can only conceal evidence.