Called again and was told by AC that they reject all seat requests from UA. Arrrgh!

Below is the text of UA's form letter response to my complaint on this matter:

Dear Boraxo:

Thank you for contacting us regarding your seat assignments for your flights. I'm sorry you are disappointed that you weren't able to get the seats you had wanted with your flight reservations. We provide seat assignments to as many customers as possible when they make their reservation. I'm sorry that we weren't able to get your preferred seats for you. We can only assign what we still have available when you purchase your tickets. Our partner airlines also hold some seats for their own use and assignment. Customers who don't have advanced seat assignments may be asked to wait until shortly before departure to be assigned a seat. This is because we hold preassigned seats until 20 minutes prior to the time we're supposed to leave. So, while you may be a confirmed passenger aboard a flight, you may be on standby for a seat assignment.

There may be times prior to travel when other passengers with preassigned seats cancel their itineraries. If this happens, we may be able to assign alternate seats for you. If we are unable to assign them at the time of reservation, we can try to assign them at a later date if they are available. I understand your frustration. Still I hope you will reconsider and allow us to serve your future travel needs.

Though we cannot guarantee seat assignments, we do try to provide you with your preferred seating. Your dissatisfaction is understood. I will give your comments to the appropriate management to help us evaluate service initiatives that will impact your travel experience. We appreciate this opportunity to respond and look forward to serving you.

Regards,

[UA CSR]
United Airlines Customer Relations