You need to be able to modify comments for some amount of time to correct typos, but if you can modify them for too long you can rewrite history. The current time limits are arbitrary but seem roughly correct, judging from the fact that users complain roughly equally about missing typos till it's too late, and people they're arguing with rewriting their comments.
It seems the assumption is that after you make a comment you are actively checking the thread for some time afterwards. Perhaps this works here because of a correct assumption about how people use HN but a time-constrained lifestyle might prevent this in some other groups.
What about letting people append to comments after the time limit? I've always thought that would be sensible: People could add to what they've said before in a visible way, but not "rewrite history".
After all, additions are already frequently marked as such (e.g., "EDIT:").
