This is courtesy of BoingBoing:

Following Cory's posts about Stephen Colbert's amazing performance Saturday at a White House dinner, many BoingBoing readers wrote in to ask:
"YouTube has taken down the videos [of Colbert's performance], citing copyright infringement. Since those videos were taken from C-SPAN, which I thought was owned by the public, who owns the copyright and could have asked for the videos to be taken down?"
(thanks, Parker, and many others). YouTube customarily removes copyrighted content at the request of rightsholders, but some troubled readers wrote in asking whether censorship or alien conspiracy theories were to blame in this case. I asked YouTube spokesperson Julie Supan, and she replied:
The Colbert videos were removed at the request of CSPAN, the copyright owner. Many of our users have inquired about whether or not the speech was considered 'public domain' and therefore exempt from copyright protection. Unfortunately, the video footage uploaded was broadcasted and owned by CSPAN.
I might recommend contacting CSPAN to better understand the situation from their perspective.