there are law against entrapment. but when fbi agents get on a chat room anonymously and send someone a link "want to see this illegal content?" now the person who could have zero interest in the matter will just want to check it out to see if it's true, for whatever reason, which could very well be to report to the authorities if it is true. but now, since the person was entrapped, the fbi can just deny the anonymous entrapment and prosecute the person for arriving on the site by their own means and drive.