And this raises the other point. If I license my software under a non-free license such as CC-BY-NC-SA, there is absolutely nothing stopping me from licensing i

And this raises the other point. If I license my software under a non-free license such as CC-BY-NC-SA, there is absolutely nothing stopping me from licensing it additionally under a different license. I can do that if I'm the copyright owner. It becomes more difficult when there are multiple contributors to the software, and therefore multiple copyright owners. You would have to obtain the permission of all of the copyright holders before you can do that. There are ways around this. For example, people submitting contributions to a project can be asked to agree to hand over copyright of their contributions, though this might not fly in all jurisdictions. Alternatively, all contributors could be asked to agree to delegate licensing decisions to the project lead team.