Couple of points:
1. This has to have an environmental factor:
I have no rust on my discs or hubs at all, but I have seen a lot of BMW's that do. The rust in the discs will be rubbed off in normal use, but the rust on the hubs / rear drum brake will obviously not be.
There has to be something which accelerates the rust, either local air quality (e.g. proximity to coast) or because of the cleaning chemicals used.
2. I have had very good customer service from BMW over the last six or seven years. Warranty problems I have had have been sorted quickly and efficiently. I have had to complain, but I usually get what I want
I think there is an issue here and if I had the problem on my car I would probably kick up a fuss. The difficulty is that your dealer is taking the stance that 'they all do that'. Plus they don't know how to fix it.
You could argue that the discs are a functional component which cannot be protected from surface oxidation. You could not argue this in respect of the drums or hubs.
You could try e-mailing BMW direct:
Tel: 0870 5050 195
Tel: 0870 5050 160