There's a paper from Stefan Savage's group a while back where they were able to estimate overall revenue of a few big spam-generating pharma/counterfeit product networks, and it looks like the anti-spam industry is vastly bigger than the underground spam-generating industry, and had no desire to shut off the ultimate source of their revenues.
There's a reason we don't see many of those emails anymore, and it's not because of any action by anti-spam companies - it's because firms whose products were being counterfeited convinced the credit card companies to shut off the banks handling payments for these purchases. (most evidently went through a couple of banks in I think Azerbaijan) Evidently all the viagra spam was coming from people who also hawked fake Gucci stuff...