"The best place to stop abuse is as near its source as possible."
Meaning: it's far easier for network X to stop abuse from leaving its
network than it is for 100,000 other networks to defend themselves from it.
Especially since techniques for doing so (for instance, controlling
outbound SMTP spam) are well-known, heavily documented, and easily put
The problem with countermeasures that would actually hurt the source of
junk heavily enough would also have to hurt "legitimate" traffic making
you an immediate lawsuit magnet. If that would not be the case, or some
larger parties feel they could stand despite this fact, the problem
would be fairly straightforward to reduce to a fraction in a few months