It's a single userland component; why would it make the rest of the system GPLv3? I mean, Linux distros ship BASH without it somehow infecting the rest of the system, why would Darwin be any different?
The real reason isn't that the system would be GPLv3. The real reason is that GPLv3 includes a patent grant clause, which many companies (Apple included) are very worried about applying to them if they ship GPLv3 code, therefore they don't.
Afaik GPLv3 is "infectious". If you use a v3 licensed component, you basically have to license the entire codebase as v3 to be in compliance. This is the reason why Apple only ship GPLv2 and MIT/BSD licensed components
