That
doesn't even make any sense. IPv4 is a contended resource, but
IPv6 is not. They're already double-dipping by charging for the
extra BGP sessions.
Perhaps, is the issue that what customers are paying for is the
ability to have multiple BGP sessions against a wider address scope
than standard p2p toward the same Cogent edge router, rather than
just paying for the wider address scope space itself?
In other words; p2p BGPv6 session = free; p2mp BGPv6 session = $$.