Nothing to do with infections? If a developer wants to use a GPLed library in a proprietary project, his choices are:
- Clean-room rewrite the library. Huge waste of time.
- Relicense the entire proprietary project under a GPL-compatible license.
I'm sympathetic to how it could appear like the license is trying to spread on its own. Obviously "infection" is not 100% accurate (what metaphor is?), but I haven't seen a better way to oversimplify this fairly unique aspect of the GPL. I'm afraid "infect" will be used until someone can think of a more appropriate term.