Well, they have made good use of code from the various BSD projects. That is exactly what Microsoft and others have done. The GPL doesn't allow that. Calling it the weakest link in open source is like calling Shaq O'Neill the weakest link in the LA Lakers lineup. They are not the same thing.
Open source implies a development methodology that is shared by both.
Free software implies a license designed to ensure the four freedoms noted above. The division between the two is often bitterly contested, both from inside and outside the communities. The holiest of holy wars are not fought over word processors, operating systems, or compilers. They are all about software licenses. Live and let license.