Some Tier 1 ISPs do not publish their peering prerequisites or peering policies. They say it does no good to publish them as they already know whom they want to peer with; they don’t need to advertise. Publishing the Peering Policy would only cause more work for them since the resulting peering requests will undoubtedly be denied.
So they protect these documents under nondisclosure agreements (NDAs) to:
If the documents are protected and come under public scrutiny, then an NDA has clearly been violated, providing more evidence that the potential peer that received the policy under NDA cannot be trusted as a peer.