Playing devil's advocate here...
There is one scenario where the content.provider is
paying the carrier as well - when the content.provider
is a direct customer of the carrier, rather than being
either a SFI-peer or a customer of an SFI-peer.
This of course goes back to the question of
depeering/transit/etc which we beat to death a couple
of weeks ago - many carriers want to get paid both by
the sources and sinks of traffic (it's certainly an
understandable, if unlikely, desire). I would just
like to point out for the record that none of the
recent depeering battles have involved any RBOCs...