Its not entrapment. Not even close, entrapment is when law enforcement make someone do something illegal they usually wouldnt do. I mean, people who view CP will view CP, they werent forced to do it.You wont find someone just go on a CP distribution site because its a good site. You'll find more paedophiles viewing the site but not regular people.
Im not completely agreeing with what the FBI did but look at it this way. The people who went on those sites are going to view CP somewhere, wouldnt it be better if you get paedophiles on one site where they're monitored? Also, just shutting down sites isnt going to do anything. Its like shutting down Piratebay or Kickass. A new one will pop up, someone probably has a backup somewhere of the site.