Right. Contributing changes to the GPL'd software itself back isn't the issue. It is the intended "viral" nature of it, that also enforces the GPL onto all code

Right. Contributing changes to the GPL'd software itself back isn't the issue. It is the intended "viral" nature of it, that also enforces the GPL onto all code linked to it. See for reference the issues with GPL-symbols in the Linux kernel, ZFS, or libreadline,