Another counterexample?

Both MySQL (now part of Sun) and QT (unless Nokia has changed this) can be proprietary
licensed from their respective companies:

http://trolltech.com/products/qt/orderform
http://www.mysql.com/about/legal/licensing/

How this is possible is that, unlike the Linux kernel which has a lots of copyright holders,
or one of many free software projects that has the copyright assigned to the FSF, the
copyright is owned and controlled by a single commercial entity. This allows them to offer two
(or more) sets of terms that run parallel. In the case of QT and MySQL, this allows them to
offer non-GPL compliant licenses for a fee.