You live in a bad neighborhood. The local Dominos Pizza, their delivery drivers having been robbed numerous times when making deliveries in your area, have decided to effect a boycott of your neighborhood. They now refuse to drive down your street because your neighborhood is too dangerous.
Is it Dominos Pizza's fault that you share a neighborhood with scum and malevolent ilk?
It might not perfectly mirror the "sharing a network" analogy, but please don't complain about the poor widdle innocent third parties
These alleged innocents have chosen to NOT perform any diligence on the NSP that will be their upstream. These innocents have chosen to engage in business transactions with, and give money to organizations that finance or support criminal operations. Anyone helping the spammers are just as guilty as the spammers. Even more odd are the network providers that use their legit customers as human shields against the spammers. Obviously they have decided the income they make from the spam operations are more important than their legit customers.
Why does everyone insist on treating the internet like it's a public resource? The Internet is a collection of private networks (and private property). Peering operates through cooperation and agreements to play by the rules.
Place the blame exactly where it belongs with a caveat emptor to boot.