Well, if you go to the site and search for 9/11 TV you'll find pages like
http://www.archive.org/details/bbc200100112023-2104 . They carefully note that it's available as a "loan (stream) only" (and note the stream is still available). I'd hazard a guess that's the only way they can legally distribute someone else's content.
The archives you were talking about, though, are presumably the source files, not available via the front end of the site. If people start downloading those then maybe the "it's available as a loan (stream) only" defence no longer applies, and so they decided these source files had to be removed.
Or maybe the answer is even simpler: bandwidth. I've been downloading CNN files overnight since these links appeared on 911blogger, for instance, and have grabbed maybe 30GB in a few days. I expect others have been doing the same, and I wouldn't be surprised if hundreds, maybe thousands of files were being downloaded last night. Archive.org must have a lot of bandwidth, but even they might be concerned if traffic increases by hundreds of gigabytes a day, in which case moving the files is exactly what anyone would do.