root can't write or modify /bin, /usr/bin, /sbin, /usr/sbin

On a debian system, I'm logged in as root.

I cannot create any files in /bin, /usr/bin, /sbin or /usr/sbin.

I can create files in /, /usr, /usr/local, /usr/share, etc, etc.

All these directories are owned by root and have permission 755.

The partition is not full.

touch /usr/bin/foo

touch: cannot touch `/usr/bin/foo': Permission denied

(and the file does not already exist)

touch /usr/local/foo

(no error)

Also, chown and chmod fail on directories named "bin" or "sbin", but not on other directories.