Google allows (allowed?) relatively free socialization on internal mailing lists, and there are serious labor law issues with saying "you can talk about anything you'd like as long as it's not company policies or politically controversial".
most companies don't have to say anything like that. the absence of active encouragement is usually enough. the "default" is not to discuss sensitive topics over company email.
Google specifically wanted to encourage open discussion of things like company plans and priorities, which at many other companies are sensitive topics.
