I haven't read the thread...but I think it's a bit like waving a stiff drink in front of an alcoholic. You can't really put this kind of undercover operation in the same category as other types if you accept that pedophiles have uncontrollable urges. It doesn't excuse the urges, doesn't excuse the crimes, but it's a sick thing to wave in front of their faces. Anyone ever seen "little children?" There's a man in the movie who's a pedophile...and by the end you actually end up pitying the guy because he's so clearly sick and he hates himself so much that he castrates himself.
Anyway...picture this...Joe pedophile is trying to resist his sickness...so he peruses the internet because he sees it as a harmless way to satiate his demons because he figures so long as he never meets or touches an underage child, he can avoid victimizing one. Then one comes along who WANTS to meet him....and the temptation is too much to bear. Only the boy or girl who is tempting and provoking them is really a cop.
It's easy to villianize these people. I was raped by an adult man when I was 13 and it shaped my life whether I like it or not. I've made some posts recently about feeling like I didn't really have a youth and this is part of the reason why - no child should have to see the evils of the world in such brutal fashion. Once a sick person makes the leap to a sick person who has committed a crime, all bets are off.
But who can say that all of these people arrested through these means would commit a crime if they were not taunted and teased by these fake underage characters? I don't know....so much gray area. I don't think there's any easy answer here.