Not all communication has positive value. 99.9% of the e-mail I receive not only has no value in itself, but the overhead of managing it, ignoring it, and categorizing it is highly negative -- and decreases the value of the valuable e-mail I receive, because I can't be arsed to check it promptly or consistently because of the overhead of the dreck. But as others point out, even charging money would only reduce spam by an order of magnitude or two, not entirely -- and since I send 1 - 10 actual e-mails a week, I only need to receive a dozen a week to never pay a penny.

My primary goal is not to send e-mail for free -- my primary goal is to have reliable, low-overhead communication with humans. Having this sponsored by spammers is a fine start, but even if I paid a dollar a year or so, that would be much lower overhead than even a day's worth of looking through spam is today (at the rate I value my time -- but even if you value your time orders of magnitudes less, the payoff is there).