That's not about technicalities so much as fact. The FOSS(GPL 3) is not void. The Mir development code is publicly available at https://launchpad.net/mir. The development is public. If Canonical decides to change the licence to a proprietary one at 12:00 PM tomorrow, then you are free to fork any code that is available up to 11:59 AM under GPL 3 if you are so inclined. That means any contributions made by any contributor will be available for any one to use it. The proprietary version wouldn't be able to accept further contributions from the community under GPL 3. Therefore, there is no stealing of one's code by Canonical for a proprietary product.
Originally Posted by jrch2k8
I hope that clears some things up.