You shouldn't shoot your mouth off about things you know nothing about. You absolutely do not own anyone's content here. There is nothing that you can put in the terms of service or the rules of the forum that can change that. Legally is it NOT yours. I don't have a reason to want to remove my content, but some people might have very good reasons for it. I guarantee you if anyone decided to get a lawyer involved, you would lose.
Again, even Facebook and Twitter cannot assert that right, even though it's in the terms of service. You can edit and delete any and all of your content there, because they have smart lawyers that are protecting their owners from themselves.