We just finished a 2 month test with eCommerce retailers. In the test, we setup an account with an online retailer and made a purchase. During the sign-up process, we made sure that the check box for subscribing to their newsletter was left unchecked. 9 out of 10 vendors disregarded this preference and auto-subscribed us to their newsletter anyway. 100% of the newsletters stopped after we unsubscribed from them. My take away is that retailers cannot be trusted to handle their own communication preferences, but since spam is so prevalent, these retailers must use 3rd party newsletter systems. These newsletter companies will advocate for your privacy and really do a good job. Mail chimp (http://mailchimp.com) is our favorite newsletter service and we recommend them to all our clients.
So the silver lining of spam messages is that newsletter service companies keep the online retailers honest.