The positive value-added is well captured (through sales conversions), while the negative value isn't. The annoyance of the sendee isn't expressed as a charge t

The positive value-added is well captured (through sales conversions), while the negative value isn't. The annoyance of the sendee isn't expressed as a charge to the sender. So it's a net-positive for the sender, even if it's a net-negative for everyone involved. It's a tough problem to solve the general case, even if some specific cases have been (mostly) solved (eg: some types of pollution).