He is NOT arguing for proprietary software. He is however advocating for projects to own their copyright, so that they can build a revenue-generating plan around it. If that plan happens to be dual licensing, so be it. Even RMS has acknowledged that dual licensed software can be acceptable, sometimes.
Has the world really suffered that much due to the availability of Qt under a commercial license, as well at GPL and LGPL?
Just to make it clear, I am not arguing in favour of contribution agreements. I believe they don't work, simply because they introduce a barrier to participation which discourages many to join, as well as the fact that most free software is developed by the "lone hacker" or collection of "lone hackers" who are not necessarily interested in building a business around their idea (and might not be good at doing so either). But the man has a vision and he's earnt the right to try his hand at it. It's perverse to twist Shuttleworth's words to claim that he is advocating for proprietary software.