The short version: RMS worked at AI lab who had a laser printer. It jammed a lot so he wanted to fix the code, but he could not because the code was non-free. His inability to fix the printer, despite having programming skill to do so, was stymied because Xerox wouldn't let the AI lab have the source code. To add injury to insult, the person at Xerox that told him no was a previous student at the same university, who basically said that he could not give them a copy because he had signed a NDA.
That is the original motivations and reasoning behind the GPL. To prevent that situation from happening and allow the programmer to fix the bug in the code.
The short version: RMS worked at AI lab who had a laser printer. It jammed a lot so he wanted to fix the code, but he could not because the code was non-free. His inability to fix the printer, despite having programming skill to do so, was stymied because Xerox wouldn't let the AI lab have the source code. To add injury to insult, the person at Xerox that told him no was a previous student at the same university, who basically said that he could not give them a copy because he had signed a NDA.
That is the original motivations and reasoning behind the GPL. To prevent that situation from happening and allow the programmer to fix the bug in the code.