1) you ban people who are rude and who you disagree with philosophically, while you do NOT ban the equally rude people who you agree with.
For evidence of the above, look at users like etherael (and his other names; not sure if he tors/vpns or if you can find them), who are raging assholes on a semi-routine basis, but who aren't banned because Libertarian BitCoin Lover matches your values. And let's face facts, you're less willing to ban people who agree with you, even if they're toxic assholes.
If you banned people who you agreed with for the same exact crimes as those you disagree with HN would be a better place.
As it stands, people who agree with you are allowed to be ruder and more toxic than people who disagree with you. This is used as a game by some of HN's worst users who brag on IRC about how it's fun to try to engage in flamewars where they don't get banned but the other individual does.
edit:
Not to mention other game that's played by a lot of folks, which is to be as big of a dick as is possible without actually using openly aggressive language. The goal there being to generate an emotional reaction while retaining some semblance of plausible deniability, because everybody knows that you won't ban them for "polite" taunting, even if it's toxic shit that can't go anywhere useful or interesting.