The problem is that the content is a co-creation. Comments aren't atoms, they're nodes in a tree that only make sense in connection with other comments. A reasonable deletion policy needs to fit this domain, and that makes it more complicated than "these are my posts and I'm taking them back and going home". It's not that we don't care about users or don't want to treat them fairly; not at all, and we go to great lengths to help.
While I agree that it breaks the conversation around others posts by allowing deletion, at the end of the day this isn't unique to HN. If i delete myself on Facebook, i delete myself, why does HN feel it's special in that it deserves to maintain conversation integrity?
pg wrote about this here: https://news.ycombinator.com/item?id=6813226. I wrote more at https://news.ycombinator.com/item?id=23623717 and elsewhere in this thread.