Where I live they start salting the roads at the end of Oct/beginning Nov and keep on salting until beginning of march.
As I stop riding when they start salting there has never been any corrosion on any of my bikes.
Some people keep riding in the winter season, and this takes a terrible toll on the bikes, no matter what brand, including BMWs.
Salt is used since it is cheap, but is terrible to steel as well as aluminum and magnesium. That is why it's banned from airports. Even aviation-grade aluminum corrodes if exposed to salt.
From what I can observe, BMW are no better or worse than any other brand when it comes to corrosion. ANY vehicle corrodes when driven on salty roads., and if kept away from the salt they all handle the wet pretty well.
However, if the bike corrodes if it has never been ridden on salty roads, that's not good and should not be accepted. But personally I have yet to see that.