Is that rule really enforced though? Should it?

I constantly see submissions here that do not use the original title. In fact, I have noticed that some/most of the time (I regularly save links from here in a personal db) the altered titles do a better job of summarizing the article than the original title. It happens to me all the time that I choose to save the link using HN's title instead of the original (better results when using it as tags).

If there is such a rule, it might merit more/finer discussion.