There are many ways to provide a legal construct that protects the interests of the unpaid contributor when copyright assignment is desired when there are business interests involved in any sort of dual license business model that may involve the option of proprietary licensing.
Shuttleworth's choice of Qt as an example of blanket copyright assignment is remarkable in its irony. When you really look at the history of it, its says the exact opposite of what Shuttleworth wants.
1998!!!!!! TrollTech enters into an agreement with the KDE Free Qt Foundation which gives the foundation the ability to relicense the Qt codebase as BSD if TrollTech stops development of the open version.
Let me make this very clear. In 1998 TrollTech agrees to an escape hatch clause which would pretty much destroy a for-pay licensing scheme by making a BSD licensed Qt available for anyone to bundle into a proprietary application if they ever walked away from the open codebase. That is a very strong, legally binding, statement of TrollTech's commitment to the external contributors. From the date of that original agreement onward, it's no longer a naked blanket assignment. Because of that escape-hatch agreement, some of the most egregious potentially damaging actions TrollTech could take are mitigated. It may not be an ideal solution for some, but as a compromise that tries to weight both business and contributor interests its extremely instructive.
2009: Nokia drops copyright-assignment completely on Qt and moves to a contributor agreement that is a based on a license from the external contributor to Nokia.
This is a good read. To quote:
"Granted, our releases have been open source, but our development model has not."
"Our goal with the new site is to make this process as simple and welcoming as possible, and thats why we will no longer ask for copyright assignment."
Someone at Nokia get's it. And I'm very sure that there are people inside Canonical that get it as well. Unfortunately, those people aren't Shuttleworth and have near zero say in setting corporate policy with regard to equitably crafting copyright assignment.