Symbolic links are readonly!?

I have tried a remastering of KNOPPIX, in which I have included
symbolic links in /usr (readonly part) pointing to /var (readwrite part).

When testing, the links' destinations (/var/...) are effectively writable if addressed directly. However, when I try to write to it via any symbolic link in /usr, I get the message 'readonly file system'. This occurs independent of the position of the symbolic link in the complete address of the write destination: even if the symbolic link points to a directory in /var, and the file is several levels down this directory.

Any clue?