You are definitely right and I was probably not clear. I'm always making my seat reservations over the TC Gold line.

Funny story about my newest seat reservation:
I have a flight to Frankfurt next week. I already booked the flight several months ago and therefore I got (the first time in my traveller-life) a cheap ticket.
Yesterday I called STC Gold line and asked for seat reservations. They told me that it is not possible to do seat reservations because it is the cheapest booking class. She explained that even for Gold Members it is not possible and also her Supervisor* confirmed that.
Today I called again (of course there was another agent) and she gave me the best seats without any problem.

* of course I got his name and I will reoprt this issue to STC Gold customer care. I hate people not serving their best customers just because they had luck one time and got a cheap ticket.