What of the harm to peaceful protesters who aren't going to be arrested? Should they be allowed to organize and plan peaceful actions without police officers continually derailing their discussions by trying to incite violence? What of the people scared away from protesting peacefully by that scary guy in the corner who keeps talking about blowing shit up? Should they be continually fearing that any expression that can be taken out of context as indicating they're advocating violence (Such as riffing off a Batman quote http://knowyourmeme.com/memes/some-men-just-want-to-watch-th... )

Which might be moot if I believed any of the groups mentioned would have committed these acts without the involvement of police officers or informants. But I don't, so.

[edit: Looking up another reference, I found a case you might find troubling.

"In the case of the Fort Dix Five, which involved a fake plan to attack a New Jersey military base, one informant's criminal past included attempted murder, while another admitted in court at least two of the suspects later jailed for life had not known of any plot." from http://www.guardian.co.uk/world/2012/mar/20/fbi-informant , more info at http://www.foxnews.com/story/0,2933,449728,00.html ]