Also, giving a presentation to thousands of people, whether at a conference or over a broadcast, is at least perceived a lot differently than leaving a comment on a message board, especially if it's a small one or a niche community, so different things are shared.
Suppose one is foolish as an 18-yr-old and posts on a forum most browsed by his friends, "Ha! I just got Lifelock and since I know you can't do anything to me when I have that, my SSN is 999-99-999. Just try to steal my identity!" Suppose the IA saves this statement. The OP would have a copyright interest in it and would be within his rights to point out that the IA has no privileges that entitle it to rehost that content, so please take it down. That is totally fair.
There is no reason that copyright law should only be usable by media conglomerates that mostly use it to stop the spread of free culture and not by private individuals trying to clean up some of their past mistakes.
The broadcaster would typically own the copyright to the video. I am sometimes given specific waivers to sign when I'm recorded at conferences and the like--mostly because rights to use material for marketing/commercial purposes are more restricted than the same material used for editorial. Frankly, most events etc. don't bother because the (correct) assumption is that people doing things in public aren't going to suddenly want to get rid of the content.