Long AS Path

The reason that a private ASN in the public routing table is an error is that the AS Path is used to prevent loops. You may have private AS 65000 in your organization and I may have another private AS 65000 in my organization. If my ASN 65000 is in the AS path of a route sent to you, then your AS 65000 will drop it, thinking it were looping back.

BTW, this is different from a confederation member AS.