One issue that companies see is that users sign-up to newsletters or the like, then forget they have done so and hit the "report spam" button when something drops into their inbox.
If such spam reports are made, then the sender's emails will be blocked by the service provider and legitimately requested email will not reach its destination, thereby affecting the relevant sender's business.
Apologies for this addition, but I would also like to note that this specific scenario is a non-issue. If a service provider blocks mail because of user requests or any other reason, it is within its right to do so. Mail server farms are private property; no sender can have a subjective right to reach anyone's inbox, whether or not said sender's profits would be affected by its bulk mail not being read. I guess the bottom line is that bulk mailing is such a tainted industry that anyone who sets up shop therein must assume that their mail will be blocked to heaven and back. -- Thor Kottelin http://www.anta.net/