Contributor contributions are a very complex issue, and it's a shame that the arguments are always simplied into black and white responses.
Unfortunately, Mark Shuttleworth is guilty of this too. The idea that people who don't sign over their copyright are just not being generous enough or are not willing to accept responsibility is disingenous and slightly insulting. To tackle copyright assignment, you have to address contributors' fears that their contributions to free software will be co-opted for proprietary gains. That is one of the core responsibilities of any large free software project.