A few thoughts:
1) I can imagine that the airbag deployment was unexpected and unwelcome. It certainly appears to be expensive.
2) 25 degrees is a 47% slope by my calculation. That is approximately four times as steep as an extremely steep public road. It is six times as steep as a very steep public road. It isn't what BMW would consider normal use, IMO. I expect that is the basis of their opinion that it is your responsibility, on top of the impact to the undercarriage. Undercarriage impact is a known cause of airbag deployment, not specific to BMW.
3) While the airbag sensor is a g-sensor, it is conceivable that it is affected by its orientation, ie when it is inclined it may trigger at a different g-level.
4) Airbags don't usually go off under 10 mph, but some have been reported to go off at 7 mph. Most airbag injuries are due to low-speed deployments, often unwanted deployments.
5) Brochures and advertisements aren't generally proof of what is considered normal use. Many advertisements have notes in the fine print similar to "closed course, professional driver, don't try this at home" or something to that effect.
6) I don't know what the 45 degree reference is in the manual, can you elaborate?
There is lots of info here:
http://answers.google.com/answers/th...39.html#answer