Marketing on the Internet is not the same thing as marketing using mailing lists or telemarketers.
Sending the same message to multiple newsgroups for the purpose of marketing is unethical. Creating an E-mail list (unless the individuals ask to be on it) for the purposes of sending "junk E-mail" is unethical.
No, it's not illegal. Yes, some people do it. But Internet users despise these people more than they despise telemarketers at dinner time. This behavior is a waste of Internet resources. Further, there are now software applications that automatically delete spams (multiple postings), meaning even if you do decide to try it, it won't work the way you expect.