It's fun to think that we're so important that the US government cares enough to intervene in our political discussions. But we are not, not a single one of us. If pg himself called for open insurrection in his next essay, no one in the NSA would lift an eyebrow or raise a finger. Until this or any community becomes known as a hotbed for muslim extremism or communist agitation we're simply not on the radar in any way. As far as hackernews and reddit are concerned, "shill" is a synonym for "someone who disagrees with me" and always will be.
I agree with the sentiment that this does not imply reddit or hackernews are subject to influence by the United States Government or allies.
I do not agree that the idea is preposterous or laughable. This is because we do know that the NSA infiltrates domestic technical groups as they did with the IETF to affect standards discussions, that they infiltrate activist groups inside the United States to disrupt them, that they are aware of social contagion theory and its usefulness in affecting public opinion, that they have done studies with at least the UCLA on viral messaging for Americans (to compare to, with and against foreign countries), that political campaigns use social targeting techniques without branding and will comment on news articles (to be 'first to post') to color conversation on hot button issues during the races, and that companies with political interests and who share a revolving door with elected office also advertise political discourse online in this way. Thinkst researchers studied how easy it is to manipulate online social conversation, news media outlets and platforms. We know that the GCHQ have JTRIG capabilities to perform internet manipulation and that there are documents from Snowden that specifically mention their use in derailing conversations on online forums. There have been reports of PR firms of private companies astroturfing reddit and others. And we know that HBGary Federal and other cyberoperations contractors for the US Government sell astroturfing services.
What we don't know is that reddit or hackernews are targeted specifically or for domestic purposes by the US Government. We have a few indications that this is done for large media outlets (recently Judith Miller, Ken Dilanian, CNN on Bahrain) in tandem with other leverage like access to officials, exclusive press passes and permission to report at the edges of no-reporting zones. Unfortunately there isn't enough evidence to be conclusive yet about the reddit/HN case as there have not been leaks that speak directly about it, so any debate in this area is bound to be speculation versus speculation.
Additionally, domestic US propaganda is now legal:
https://www.techdirt.com/articles/20130715/11210223804/anti-...
Further, sock puppetry is an established tactic:
http://www.theguardian.com/technology/2011/mar/17/us-spy-ope...
http://mashable.com/2011/03/17/centcom-social-media-personal...
http://www.fbodaily.com/archive/2010/06-June/24-Jun-2010/FBO...
So the only question is, specifically which sites are targeted and to what ends. If the metadata shows that HN or your favorite sub-reddit has out-sized influence on matters of national concern, then they're probably targeted.
"Incredibly costly" + "I don't see how they would benefit" is only an effective argument against individuals and businesses whose continued existence depends on not wasting money. The state runs on taxes and executive orders.
You may not realize that one of the non-publicized goals of recent executive administrations has been to keep the official unemployment rate of war veterans low enough that it stays out of the public consciousness. This has been accomplished in large part by steering them into make-work jobs with strict citizenship or clearance requirements along with hiring preference points for military service.
The government probably does not care to intervene in our nerd talk, but it can afford to, and if that provides a minor political benefit beyond sticking loyalists in dubious, relatively-high-paying desk jobs, then so be it. If I were to attempt to promote state interests with respect to encryption and network security, HN is certainly one of the sites I would pay my subordinates to read and influence.
Don't assume that just because you think it is stupid and pointless, no one is actually doing it. That doesn't mean that anyone is, but you can't realistically argue that everyone is not.