WhyGPL

From APIDesign

GPL (the General Public License) is viral - software based on any GPL library needs to be licensed as GPL as well. This is good from the global perspective of open source projects. The more GPL open source libraries there is, the more GPL applications there will be.

The virality is also good from the point of view of the library vendor. Every application based on such library needs to be available publicly. This is good for promotion of the library itself (you would not believe how hard it is to find all users of NetBeans Platform now when we desperately need to show it is useful and used; its license is not viral and thus many usages remain unknown) as it gains more visibility, it makes the developer prouder (obviously I rather use software based on my library than other), and also it increases the possible contributions back to the library (as one can easily take useful pieces of code build upon the library and backport them back to the base).

However not every user the library wants to release own code under GPL. It may seem that in this case the virality of GPL becomes an unsolvable problem. Surprisingly, the opposite is true. If the author of the library is open to such solution (like me in case of my Netbinox project), it can offer an alternative licensing mechanism. The users can then buy right to remove the virality - e.g. to incorporate the library into own application without releasing own code as GPL.

Thus licensing libraries under GPL (with the option to remove virality) seems beneficial for all involved parties. The other details are available at wikipedia::GNU_General_Public_License.