Not just BMW - they are virtually all guilty - a crying shame. Literally one thin top coat direct to bare metal on most bike frames. A total joke. At least a basic zinc primer would help a ton but they simply don't want to.
All you can do is Dinitrol the insides as much as possible, clean fully and XCP it from new and rinse/wash/re-apply and fight a battle that nature (and the road salters) will always win eventually.
When I had my Rt all the Tupperware hid the corrosion, at that time the Honda st1100 also had a bad reputation for hiding corrosion under the plastic with an added bonus they would rot the exhausts, expensive ! and BMW owners would gripe about discolouration to the headers

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not just BMW with crap paint , ok it’s about 1/2 the price of a gsa.