I think it depends on the country. In Germany, everything you write is automatically copyrighted, unless you explicitly waive it. In the US, it's the other way around, you have to explicitly state that you want copyright (can somebody confirm this?).

I'm not a lawyer, but I guess a German posting on Hacker News effectively waives their copyright by sending their comment to the US, where an US company then publishes the comment on a US server.