The former is abhorrent and should be a big scandal, but it won't be. Snowden showed that Americans simply don't care. Given that signal, why stop?
The latter is actually good, in general. LEOs doing actual police work, joining groups that might be extremist...that's the online equivalent of undercover work, and I think it's good when it's human work.
So, basically I agree that proactive, general IC infiltration of back-ends is wrong, but I think its already happening and won't stop without change of sentiment. LEOs can and should do more human investigation, join groups, get involved, see what they are about - and then leave them alone if they aren't a physical threat. Obviously this can be super problematic (e.g. the FBI vs MLK) but the only alternative is to just sit there and wait for an attack. That's no bueno, either.
As an aside, if lack of critical thinking skills sets you off easily, you are in for a rough ride. It's particularly dangerous because directed at others, it gives you clout in with the wrong people, and directed at yourself contempt can lead to self-loathing and depression.
I want to send you a repost invite for https://news.ycombinator.com/item?id=34556378.