Can you elaborate on this? What exactly do you mean with unrelated source and might you give a concrete example what a company could NOT do if this code would be GPL'ed.
The readline library for example. If I want to use it, I have to GPL my whole program. I probably might not have modified readline at all. Or any other GPL software which gets somehow linked together with my code. Just watch the controversy whether ZFS kernel modules may be delivered as pre-compiled binaries (sources are fully available). The output of compilers and parsers which are GPLed, also are implicitly covered by the GPL, unless there is a clear exemption, like with GCC.
While GPL software which is used as an entity, like Linux itself or applications like Emacs are fine, any closer contact to the software you are trying to sell is problematic.
It is in the interests of a minority of programmers, themselves a tiny minority of the general public. Let's not get completely overblown about the stakes here.
The only reason I use the GPL is to safeguard the general public's rights to modify and distribute my software. If anything, I'd argue that releasing GPL software works against the author as it makes making money off it substantially more difficult.
The general public still retains the right to modify and distribute your software with a license such as MIT. Your software does not cease to exist once megacorp uses it for a product. The guarantee of GPL is that megacorp is now obligated to share their modifications (but practically, to go find another library or write their own).
You're missing the forest for a single tree. The obligation to redistribute changes under the same license is not the only reason to use the GPL. The license's safeguarding of end-users' right to fix and modify GPLed software is a far more important reason, and is the reason why I and many people like me choose the GPL.
I don't want to restrict developers and corps. After all, I am a developer who wants control over my hard work. However, I also don't want to erode the rights of end-users - of whom I also am. I don't want to end up in a situation where I can't fix a bug in my own software because a corporation wont let me.
