My code, outside the day job, is GPL. Why should I let others commercialize it without giving anything back? But I want to allow noncommercial use for everyone, and commercial use for those who will contribute their changes.
I'm not saying everyone should have these preferences - in fact I admire the generosity of anyone who donates valuable IP to the public domain. It's just that the title is "Why your next project should be public domain", yet the author does not explain the motive for this type of licence over others.