Freedom
It comes down to your definition of freedom. The FreeBSD folks believe that any restriction of rights is the opposite of freedom. To them, software should be _completely_ free, including the right to take the code closed source. If you want to grab the FreeBSD code, change the name and issue a closed source version, that should be your right. It's like Voltaire's old quote, "I do not agree with what you have to say, but I'll defend with my life your right to say it." As a result, they've been trying to purge the GPL from their code base for quite some time.