I don't know the specific reasoning behind it. It's not specific to github. We aren't supposed to look at any reference that isn't provided by the candidate. This includes linkedin, facebook, etc.
It's most likely an overly broad means of protecting against employment discrimination.
Considering that much of what's on people's Facebook pages is against the law to even ask about (in some jurisdictions) it's probably a wise choice. Stick to what's provided by the candidate. I provide a link to my LinkedIn page on my résumé, but nothing on it touches on areas I know would cause problems. Most of my coding is in private projects so I generally don't bother with listing my GitHub.
For example, I know that where I am it's against the law to ask if someone is married during the hiring process and that's pretty clear on many Facebook pages.
It's most likely an overly broad means of protecting against employment discrimination.