2007 Sun Microsystems licensed its Java framework under the GPLv2. An important question arise from this fact is: Why have a big corporation choose a license with a strong copyleft like GPL instead of a permissive license like the BSD license?
This fact is inasmuch curious because the GPL is supposed to be a "socialism style license". But successful corporations are rational actors and interested in gaining but not decreasing their power and influence. So truly "socialism style licenses" are not really appreciated in these areas. Therefore the GPL was resp. might be in fact the best open source license Sun Microsystems could choose (among others) for OpenSolaris and their Java framework. This depends on the non-permissive nature of the GPL which license agreement is more than five times as long as the BSD license agreement and the GPLv3 is even longer. In its juridical overhead the GPL defines what is not allowed and constrains in this way the freedom of action of every individual using GPL licensed software. Hence every company which licenses software under the terms of a GPL-like license gives not as much power away as a company which doing the same according to BSD-like license terms. According to the nature of GPL, software forfeits a part of its overall power potential and everyone who possess this software in terms of the GPL gains not as much power from this software as somebody who has exclusive or BSD defined rights on this software. The power potential which is get lost due to the terms of the GPL, turns into a kind of virtual power which is possessed by nobody but prevents everybody from claiming any exclusive rights on this software. A main advantage for companies licensing their software under the GPL is that their power loss is not as big as supposed to be because the company keeps its leading role in development and extension of the given software. Furthermore the company can outsource expensive testing and debugging to the OSS community. The already mentioned virtual power potential of the software is the only part of the primal software power potential the company loses but nobody else obtains (and which cannot be obtained exclusively -- according to the GPL). This is exactly this kind of power which has be protected actively against product piracy -- the so-called "intellectual property". The GPL also excludes third parties from claiming any exclusive rights on the software. But this "third parties" must not necessarily be other companies but also individuals which are forced to use the GPL for their derivative works based on GPL software. This is the main and most important side effect of the GPL -- its viral nature: Derivative works must be licensed under the same terms containing which is the key principle of strong copyleft.
Permissive licenses like the MIT/X11 and the BSD license aspire another way. Their key idea is to give away the whole power potential of the given software by claiming that you can do whatever you like as long as you mention the original author in the copyright notice.
I was incited to write this comment by the announcement of Trolltech to release their Qt framework under the GPLv3 and the positive feedback from the largely ideological motivated naive nerdy OSS community. For many of these people the GPL is the synonym fro so-called "free software" and their preferred open source software license. At this point I want to clarify that "free software" is not the just another synonym from "open source software". Free software is open source software but open source software must not necessarily be free software.
My final question: Is a system where you are forced to be "free" according to the intrasystem calculus definition of the term "free" (as it is determined by the legal GPL calculus) a genuinely free system?