Naturally, people do not like bearing these costs, but the principle at stake is not obvious. To characterize spam as a trespass seems odd, especially since the law does not treat paper in one's physical mailbox that way. Perhaps uninvited communication is simply the price you pay for being connected to a network.
A more plausible approach is to view spam as a contract violation, since ISPs commonly have policies against unsolicited mass mailings. But spammers simply ignore these policies, and because they readily switch identities it's hard to stop them from sending e-mail to an ISP's customers. This willingness to flout the rules does not bode well for the success of anti-spam laws.
The hucksters and fly-by-night operators who represent the core of the spam problem are not about to go legit by including "ADV" in their subject headings, accurately identifying themselves, or providing genuine opt-out mechanisms (let alone by limiting themselves to people who have explicitly opted in).
If the experience with 30 or so state anti-spam laws is any indication, catching and punishing the scofflaws will be difficult, especially if they use foreign servers. So while the government does have a role in fighting spam (especially in punishing fraud, which is already illegal), our expectations should be modest.
Technological fixes, though far from perfect, show greater promise. Francine's ISP may have let those "weird and disgusting" messages through, but it generally does a very good job of blocking inappropriate material. My anti-spam software takes some effort, but it filters out 150 to 200 unwanted messages a day, leaving me to delete just a few come-ons for pornography, penis enlargement, and amazing investment opportunities.
It may be unfair that ISPs and their customers have to take these steps, just as it's unfair when somebody drops litter on your lawn. But that doesn't stop you from cleaning it up.