> Why are full disclosure policies recommended by security researchers?
This is still a somewhat controversial issue. Coordinated or "responsible" disclosure appears to be advocated by most security researchers. Meanwhile, the full-disclosure mailing list still exists, but the activity on the list is far lower than it once was.
> Why is open source healthy for security in general?
This is also controversial. Open source software has its fair share in-the-wild 0days. The "many eyes" theory only works if there actually are many friendly eyes, and so far, it seems like that is not true. Nobody does this crap for free.
> Why is "security through obscurity" frowned upon? Because it is important to be able to discuss threats in order to better protect yourself from them.
"Security through obscurity" does not come from the "information must be free" community. The phrase comes from the idea that you should assume that your adversary knows everything about the defenses you have set up.
A gun not being fired is not dangerous. A deadly virus that is contained is not dangerous. Ideas are seeds; action is impossible without them. All you're doing is pushing the responsibility one step further; instead of preventing the spread of ideas we have to prevent their effects. Now instead of dealing with the cause, we have to deal with the symptoms.
A meme like "immigrants are the source of all our problems" is a dangerous idea, because of the actions it can directly cause. Preventing its spread is just good mental hygiene.
Cults are another place where ideas are most certainly dangerous; but this time for the individual. So is any religion/ideology people are willing to die for.
Most ideas are not dangerous. Also, I might be completely wrong; if I lived in Galileo's time, I might be claiming heliocentricity is a dangerous idea.
You're missing the parent's point. An idea not being acted upon is not dangerous. The idea of splitting atoms is not dangerous and the atomic bomb is not dangerous: An atomic bomb getting built (the idea being acted upon) is dangerous.
Why are full disclosure policies recommended by security researchers? Why is open source healthy for security in general? Why is "security through obscurity" frowned upon? Because it is important to be able to discuss threats in order to better protect yourself from them.
Burying your head in the sand is just a shitty tactic for dealing with danger. Being able to discuss controversial ideas is extremely important, whether it's to better protect yourself from them (eg. extreme-right ideology) or to independently research facts hidden from you by people who don't have your best interests in mind (eg. north korean censorship).
And let's be honest here, from what I've seen of my (left leaning) twitter feed, Sam is being attacked extremely harshly for his "controversial" post. If people here think that the article is wrong, this is a terrible way to prove it so.