It's more or less random which comments we "choose to take a stand over". If we see people breaking the rules, we ask them not to. There's no implication of a total ordering, or any hierarchy of abuse.
The idea that if comment X says one thing and Y says another and we reply to X but not Y, that means we're endorsing "violent police responses to the homeless", is... not valid. It's not possible for us to see everything, even in the same thread, let alone all threads.
If you see particularly egregious comments that haven't been moderated, you're welcome to point them out to us and we can take a look. hn@ycombinator.com is the best way to do that.
