The more massive the public-good project, the greater the incentive to abuse it. And the Internet is nothing if not massive. Price Sensitive E-mail is not like paper mail. Every single piece of mail has to be printed, stuffed into an envelope, addressed, and stamped. Only then will the post office attempt delivery.
Marginal cost is the price of sending the next piece of mail.
On the Internet, you don't have to pay the entire cost of transporting each e-mail message or newsgroup posting. Someone else pays to move it around, and everyone who gets a copy has to pay to read it. More importantly, the marginal cost is almost insignificant. It's as easy to send a message to 1,000 people as it is to send it to one person. That's because each person who receives an e-mail message pays the bulk of the cost of delivering and storing that message.