1. The philosophy of Usenet
In the beginning, Usenet was largely confined to educational institutions such as universities and colleges, and to research companies and other commercial enterprises with Unix machines on-site. It has now grown to include millions of users at commercial sites such as America Online and at companies around the world involved in every sort of business imaginable. Nevertheless, many of the customs found on Usenet today have their origins in the days when Usenet was very small and most Usenet sites were universities.
Advertising is widely seen as an 'off-topic' intrusion into the discussions of any particular newsgroup (newsgroup is the Usenet word for discussion group or bulletin board). Each newsgroup has a specific set of subjects it is intended to cover, and in order for newsgroups to function as effective discussion forums, it is important that people stay 'on-topic'. If everyone disregarded the particular topics each newsgroup is intended to cover and simply posted whatever they wanted wherever they want, the entire system would break down.
Due to the decentralized nature of Usenet, there is no one person or body which can "enforce" the custom of staying on-topic. It falls on each user to help preserve the culture of open discussion and free speech that Usenet has come to embody by not posting off-topic material.