Having worked on many cars over the years, a slight amount of rust on metal brackets and supports under the dash is not unusual. Since they are inside the vehicle, they often are not coated and, to keep good grounds, are not painted either. Once they get this light coating of rust from atmospheric moisture, it tends to stop, with the rust actually providing a bit of protection against further rust.
If it bothers you, take it to the dealer by all means, and perhaps check out some other new cars on the lot.
But I myself would not worry about the stuff under the dash. However, any rust you can see when the glovebox is open, is not right and one should not expect to find rust inside the glovebox. This is something the dealership should correct.