Because I myself feel that this is such a good and reasonable way of hiring people, I wouldn't want to work for an employer that wouldn't let me do open source work.
I mean, it's also because I consider open source to be the public sphere, and I really do believe in software freedom. Maybe my professional metaphor would be the coder as a writer. Of course I want to see your published work.
I would guess the environment candidates are selected from influences these numbers significantly.
And you're right, I'd understand that in some of these environments, OSS isn't necessarily considered fun or relevant.
Then again it's all speculation.