The two issues are intertwined because whether or not default is tied to
peering depends on which sort of media your public exchange uses. A FDDI
exchange point will allow anyone to point default at anyone else without
any explicit BGP peering or FDDI configuration, but an ATM NAP requires a
permanent virtual circuit to be established between two peers before traffic
can flow and this requires the consent of both peers. Therefore, on an ATM
NAP, you are not subject to default-abuse by non-peers.