These marketers often allude to these being quantified mailing lists, but as we all know many aren't. A person who is new to the net may not know the differences in mailing lists. If you sign up for something on a site, the owner may feel that you may be interested in something else they have to offer. After all you asked for the first one. Now for the serious side of spamming.
Many people complain about it, just like they do junk mail and telemarketers.
However, most of the time we have brought it upon ourselves. Of course if the truth be told, most of us at one time or the other has found something they would like by these methods. It's amazing if we use it, we don't call the sender bad names, but if we don't use it the sender is a low life. Because they have found it to be a profitable way to market. Enough people buy, that it pays for it's self.