Any other way would mean the government monitoring our browsing history, and/or our ISPs doing it (not to say it isn't already happening). Major breach of privacy.
Who cares. Its pedophilia. Its like the FBI hiring a hitman and him actually trying to kill someone. Sure its entrapment, but I don't think a potential murderer should get any benefit of the doubt; same for a pedophile. You don't accidentally stumble onto child p*rn sites.
Sometimes you have to do things like this for the greater good of the public -- they can catch a great deal more pedos using this method than they would through traditional means.
Geez, I don't think I'd go that far. Pedophiles still have rights to things like fair trials and protection against illegal searches that should be defended. The FBI should not be engaging in entrapment stings to catch pedophiles or murderers or terrrorists. But, I just don't see how you can reasonably call this bust entrapment. Oh, and the FBI shouldn't be hiring hitmen either.