Sounds like a misfeature to me: lots of website use it to display the
right content depending on the OS the user is running (tutorials,
downloads, etc.).  Disclosing the OS family the user is running is
hardly a threat to privacy in my opinion.

Besides, isn't it deceptively advertising the dominance of a non-free
OS?  Doesn't sound quite GNU to me...

--
Pierre Neidhardt