cpoole -- well it is not normal and it is not normal for a BMW. If you actually read this thread you'll see that it happens to other brands as well. I am guessing that your attribution to BMW as their "answer" is slightly tongue in cheek. But taking it at face value -- you need to get in their faces and remind them that normal wear never includes a fire in the cabin. They will make it right.
Its kind of like telling youthat its normal for the gas line to the engine to wear out as all of that "detergent" gas goes thru it over the years. Then the gas starts to leak and it comes in contact with the hot engine parts and the car blows up. Just normal wear, ma'am.