Unfortunately some members are Tweeting and Recommending these pages, which results in errors when doing so due to the URL not being publicly accessible.
It also makes the Facebook Recommendations widget look less than perfect due to broken logos and incorrectly formatted URLs
In addition, any shared content won't be available to anyone not logged in which will most likely just frustrate Facebook and Twitter users.
A conditional to check if the Page or thread is not accessible to guests would be useful to avoid these issues.
Or failing that, a simple checkbox which can be applied per node: category, forum, page.
The same also applies to private nodes.