RMS's rationale is to protect the freedoms of the end users of the software: The freedom to run the software however you want, the freedom to modify the softwar

RMS's rationale is to protect the freedoms of the end users of the software: The freedom to run the software however you want, the freedom to modify the software however you want and the freedom to distribute your modifications of the software to whoever you want. The rationale is to ensure all users of GPL software have those freedoms. The only restriction is that you cannot remove any of those freedoms from any users of your derivative works. That restriction is enforced by the license. For example, if you create a derivative work, as long as you keep using it only yourself, you're not required to give the sources to your modifications to anyone (you are the only one using it and you have all the freedoms). However, if you wish to share your derivative work with someone else (which you always have the freedom to do), you cannot limit that user's freedoms. You cannot, for example, refuse to give them the source code because it would take away the user's freedom to modify the software. You also cannot attach a proprietary license that would remove the user's freedom to distribute copies of the software to whoever they want.