It to those that imply that Plone going the GPL route is somehow stealing that I would add this. The only possible theft of Free Software comes from placing restrictions on it that remove the rights of others to enjoy, explore and learn from the software the same way you did. The license ensures that at each step of the way its distributed with the same freedoms extending to each recipient.
This is complicated when mixing GPL software with other software, obviously. In the cases in which you'd mix with software that would restrict the rights of its consumers, such as mixing with proprietary software, the GPL is pretty clear. Its in the case of mixing with other Open Source software products that things become less clear. If both products intend for example for the works to be distributed with the source always available but define different notions of what makes an aggregate work covered by the distribution terms of the license things grow increasingly complex.
Rather than detail various cases, I'd say this. Its important that software projects define the intent and understanding with which they use the license, and its far better to ship with known, better understood licenses (even with legally binding addendums and exceptions) than to invent your own. The Plone Foundation has a responsibility, as owner of the Plone IP, to more adequately define our intended usage, derivation model, etc, that we can better protect it and the rights of the software's consumers.