Just to clarify - the police department didn't create the site from scratch. They busted the original owner of it, then took over the site 'as is' and ran it for a while longer to gather evidence of other users, then shut the whole thing down.
No different from cops everywhere knowing where a drug shipment is landing, then setting up a sting operation around that to see who comes along to collect, and then pouncing. (Rather than intercepting the drugs mid shipment or before it leaves).
No different from cops everywhere knowing where a drug shipment is landing, then setting up a sting operation around that to see who comes along to collect, and then pouncing. (Rather than intercepting the drugs mid shipment or before it leaves).