If I use a compiled program/non compiled source code which is under a license that everyone has the rights do use and sell (Eg: GPL) and no one can deny any person to use or sell it and then I want to develop an extended feature for it/document/presentation/compiled version for it to be sold for money, to be available only to the person that payed for this premium feature - Must I release this premium feature also as GPL ? If I must than it means that the person that just payed money for this premium feature can distribute it or sell it too or do whatever he want without paying me further.
If someone develop a premium feature - how can he protect his premium feature for further free distributions that will eventually stop him of getting pay for his premium work. If this premium feature will be available to all - then why would someone want to continue paying for it ?
For example: If I develop a wordpress plugin. (Wordpress is GPL). I develope a free plugin under the GPL too which everyone can take and do what they want with it (Almost), then I develop a better version of this plugin for money. Must it also be GPL ? If it does that as I wrote above - everyone can do whatever they want with it and I won't have the ability to continue selling it for a long time.
What is the solution for that ? Thanks !