The US Copyright Office has issued a statement saying that it would only issue Copyrights to humans. PETA objects to this, having just filed a suit to grant that macaque monkey the Copyright for the selfies it took back in 2011. The struggle is real.
Here's a 2008 New York Times piece on Girl Talk. Whether or not his fair use argument would hold up in court, it seems clear that music labels aren't particularly interested in suing him for his all-sample approach to music creation.