The volume of traffic flow in each direction is irrelavent.
In all cases where we're talking about *peering* (not transit) the source
or destination of the traffic under consideration is someone from whom you
collect money to provide service to.
You are *obligated*, by ethic if not by law, to provide the best service
possible given your installed infrastructure and capability to do so.
Asking a *third* party to pay you for something that you've *already been
paid for* is unethical in the extreme and, to the extent that providers are
trying this in the marketplace, they should be exposed to public scrutiny.
These kinds of extortionist tactics need to be brought out in the light of
day where everyone can see them -- especially the customers of the providers
who are doing these things.
Only through a full public disclosure and discussion of this practice will
those provider's customers come to understand that their provider, on an
ethical level at minimum, is *not* providing to them what they think
they're paying for.