Quoting the Preamble:
"To protect your rights, we need to prevent others from denying you these rights or asking you to surrender the rights. Therefore, you have certain responsibilities if you distribute copies of the software, or if you modify it: responsibilities to respect the freedom of others."
and again:
"...if you distribute copies of such a program, whether gratis or for a fee, you must pass on to the recipients the same freedoms that you received. You must make sure that they, too, receive or can get the source code."
Meaning that using GPL'ed work with proprietary work is *viable* only if proprietary work is licensed under a GPL-compatible license.
I believe you are distorting my statements. The terms required by the GPL do not apply to the GPL'ed portion only, they apply to the entire work:
"You must license the entire work, as a whole, under this License to anyone who comes into possession of a copy. This License will therefore apply, along with any applicable section 7 additional terms, to the whole of the work, and all its parts, regardless of how they are packaged"
Licenses like GPL are defined *viral* or *copyleft*, meaning that they call for distrubution under the *very same terms* for any derivative work.
Quoting RMS
?If you include code under this license in a larger program, the larger program must be under this license too.?
The license 'enforce' the obligation to license a derived work under the same terms. And that is the reason why GPLv2 and GPLv3 are not compatible, since they both would require to have the larger program released under each of them, which is not possible.
Al