Same problem for Java-SDK and other programs that can otherwise normally be downloaded and installed without charge by end-users. But their license design would make the CD non-distributable.
The software on Knoppix is not entirely 100% GPL (the collection as such, and the Knoppix-specific programs are, though), but all packages included in the download-version should be "freely distributable for non-commercial and commercial use". There are also some "non-free" Debian packages included which match this goal. For being sure that download and usage of each individual software package is allowed and legal in your individual country, please read the package licenses and ask your local lawyer.