Because recognizing these differences is the first step to marketing online effectively. When you understand that the network is theirs and not yours, you understand why opt-in email lists are the only way to do things and not just a fleeting fad. You understand why the concept of permission marketing is a business necessity, not an advertising fancy. You understand why making the protection of privacy on your site is an absolute imperative. And when you understand those things, you get to see that it's not enough just to change the way you market - you also have to reconsider the way you do business.
Conventional businesses attempt to control, carve up and own the market.
Segment by segment, niche by niche, individual by individual. Unconventional businesses succeed in letting go. They don't fight to acquire and own the network. They voluntarily relinquish any illusion of control. They don't try to exert power over the tens of millions of individuals online.