But, that's not true. The big online services have metered rates, and so do many ISPs. Furthermore, in some major cities in the U.S., all phone calls, even local ones to an ISPs dial-in modems, are metered.
On top of that, the spammers are exporting the problem to Europe, Australia, Asia, South America and Africa. Many countries around the world have no flat-rate services whatsoever, and in Europe, for example, phone rates are often much higher than in the U.S.
No matter how you stretch to justify it, the recipients of spam are getting the shaft from spammers.