I believe that the goal of GPL is that everything is GPL'ed. If everything was GPL'ed, then there wouldn't be a problem. Using their software IS free, but you're forced to share what you've done. Overall, the more people who work under GPL, the better GPL gets. That's not true with most other licenses.
As it is, a divide has been created. Kinda like a Venn Diagram, except that the circles don't overlap. On one side is Commercial, which doesn't play with others. On the other is GPL, which doesn't play with others.
I personally go with the public-domain license for most of what I do. My principle is, I don't care if others use it. If they want to give me credit, that's nice, but I'm not going to force them to.
On the other hand, I've released some software under a BSD license, which is personally my favorite license.