From the article: "How does spam differ from legitimate advertising? If you enjoy watching network television, using a social networking site, or checking stock quotes online, you know
that you will be subjected to advertisements, many of which you may find irrelevant or even annoying. Google, Yahoo!, Microsoft, Facebook, and others provide valuable consumer services, such as search, news, and email, supported entirely by advertising revenue. While people may resent advertising, most consumers accept that advertising is a price they pay for access to content and services that they value. By contrast, unsolicited commercial email imposes a negative externality on consumers without any market-mediated benefit, and without the opportunity to opt out. Read more
