Your logic is flawed. Your logic says that a child must be raped before we can stop the crime of rape. If you threaten the President's life, they will not wait until you pull the trigger to arrest you. Conspiring to commit the crime is a crime. Clearly these guys were there to commit the crime.
Entrapment.--
(1) A law enforcement officer, a person engaged in cooperation with a law enforcement officer, or a person acting as an agent of a law enforcement officer perpetrates an entrapment if, for the purpose of obtaining evidence of the commission of a crime, he or she induces or encourages and, as a direct result, causes another person to engage in conduct constituting such crime by employing methods of persuasion or inducement which create a substantial risk that such crime will be committed.
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Conspiring to commit entrapment is a crime.