Making backup copies--or any other fair use scenario--should be legal. However, the discs have some copy protection on them, and the DMCA makes it illegal to basically even think about circumventing copy protection. So while you're within your rights to own a copy, it's illegal to attempt to make one, essentially. It was a moderately clever way to completely circumvent all the reasonable provisions of existing copyright law.
At least that's how I understand it, and it's why the DMCA is one of the most preposterous and insulting pieces of legislation in the US. It's why you can't even legally LINK to a site discussing methods of bypassing copyright without running the risk of getting sued by the RIAA or MPAA or whoever, as was the case with a major website that was writing about software to bypass video copy protection.
