I think I may found the reasons for the junk e-mails.
Under the E-mail and Communication Preferences page of the "Update Profile" section on United.com, we have been asked to checked the following five boxes if we want to receive future e-mails from United:
_______________
1. [B]Domestic and International E-Fare
Special discounted airfare offers and other travel-related offers
2. Mileage Plus Communications
Mileage Summary, Mileage Plus Program Updates, United and Mileage Plus,
Partner Communications and Information, including My Mileage Plus.
3. Mileage Plus Partner Communications
Valuable offers and information from Mileage Plus partners.
4. United Offers & Announcements
Special service, product, or promotional announcements.
5. TedClub
Exclusive travel related offers and more from Ted's free destination club.
____________________
I imagine our e-mails have been sent to five different independent databases that don't share information with each other. For example, I had received an e-mail from Ted asking me to register to become a M+ member a few months ago.
Look at 2 and 3 above... isn't 3 covered in 2 already?
I agree, it is kind of stupid and waste of UA resources.