(note: this is different from Is it inappropriate behaviour to post links?; I am now asking for a debate on the existing policy, rather than asking whether the policy exists. I would be fine if this is closed, but only after a moderator confirms what tpg2114 says below.)
Kyle Kanos's answer to the forementioned thread claims that it is not allowed to post links to other Q&A sites outside the Stackexchange network, and that it's up to a moderator to suspend a user for doing so. While he has not been able to cite the link to this policy, no moderator has corrected him, and his answer is gaining upvotes, so I'll assume that this is indeed policy.
I feel this is a bizarre policy to have. What's the reason to maintain such a policy? Is it a network-wide policy, and is it irrevocable like "Be Nice"?