I disagree with this, and don’t think it’s correct unless you make special TOS which would try to achieve that. No offence, but this seems to be the “dictator way” (or at least the childish way)
In most countries, every author has automatically a copyright claim on everything he creates. So every post is legally owned by its author (there may be group ownership regimes for group works, and a topic may fall under this). In this view, OP’s question is relevant as the topic creator would indeed have some copyright on his title. Now, a topic title is quite “generic”, and I guess you could argue nothing is done to the title, but it’s only replaced by another one. Thus, no copyright problem.
Copyright would especially be if you reuse the content rather if you modify it. And in case of reuse, it is content which was made publicly available for free, so I guess, it’s fair to be able to quote it, but with credits given to the author.
As for altering the work of someone, I guess some legal conflict could arise if it isn’t defined in the TOS and you mess with the wrong person. There may be some legal aspects often totally disregarded when it comes to forums.
I really don’t think you can say what I’ve quoted at the beginning of this message.