Ah, I get it now. And now I think I understand why others just put dots or something.
If you use the "Post Reply" link, then the subject line doesn't autofill. But if you use the little icon to the right of the subject line of the message you're replying to to respond, it autofills the subject line with Re: at the beginning.
Perhaps they should mimic the autofill for the "Post Reply" link as well.