Is it possible in the United States for a dark web site operator (or potentially anyone involved in an illegal activity) to create a contract that legally binds a law enforcement organization to grant preemptive immunity against any illegal activities that might occur?
For example, upon sign-up the site might present a EULA or similar that says something to the effect of:
"By signing in to this site, you, as a representative of any law enforcement agency, acting as a representative of and on behalf of your respective agency, do hereby grant the site operator and any affilates of this site perpetual immunity from civil or criminal prosecution for any and all activities associated with this site, including indemnification against damages for ..."
Is there any legal predent anywhere in the United States where a person has been encouraged by a law enforcement organization to commit a crime under protection of immunity, possibly in pursuit of some greater good (justification: it takes a criminal to catch a criminal)? Could such a precent be used as a defense in conjunction with this agreement ("They signed the agreement Your Honor, just like they did with Homer vs. The City of Springfield, 1985")?
If it isn't possible, what law(s) prevent it? If it is possible, what would such a contract look like (note, I won't be the one to test this, just curious...)?