It's been a while since I posted a longer explanation of what we're doing when we do this, so maybe one would be helpful.
The core issue is the word primarily. It's one thing to occasionally comment on the politicized topics that naturally come up on HN. That's what most users do and it's fine. But there's a different class of account that exists primarily to do this, pushing a political or ideological or national agenda. That is what we don't allow, as the site guidelines are careful to say. We have to draw a line somewhere, in order to prevent flamewars from taking over HN entirely, and experience has shown this 'primarily' test to be the best place to draw it.
The reason we draw the line this way is that there is a bimodal distribution of accounts and the two categories behave differently. People in the first group are motivated by intellectual curiosity, so post on a wide range of things and tend to post curious, factual, open-minded comments. People in the second group are fighting a war. That destroys the curiosity HN exists for. Indeed, that's the first thing it destroys, because the point of political battle is not to curiously meander, but to blast enemies. This site is for curious meandering.
This second type of account often comes armed with pre-existing talking points and links, which can look like contributing to good conversation, and sometimes add good information. But their fixation on an agenda and the intensity with which they seek to destroy other views instead of—let's say—dance with them, make their cumulative effect on HN not just negative but toxic.
Figuring out how to moderate this kind of thing without excluding politics altogether (which is undesirable and impossible anyway) is a challenge, and the observable difference between these two kinds of account turns out to be a useful heuristic. So when we notice an account that shows the telltale signs of being agenda-driven and guideline-breaking, we ask them not to do it. We try to explain why, and we ban them if they don't change.
There's an aspect of this that isn't publicly visible. It's not uncommon for people we've banned for abusing HN in this manner to create new accounts to continue doing it. So sometimes, when we bring this issue up with a new user, we're looking at more of a track record than might appear.
None of the above implies that an account is wrong in its views or that the topic at issue is unimportant. On the contrary, it's almost always very important and legitimately intense. That's actually why we have to moderate these forces on the site-in the literal sense of 'moderate'. Otherwise they would dominate everything, which would turn HN into a very different kind of forum. It would also quickly kill it as a place for thoughtful conversation.
More on this at https://hn.algolia.com/?sort=byDate&dateRange=all&type=comme....