My logic would be that MS pays only for 300% of miles to BD since BD technically converts the other 300% status into destination miles after 55K, which is purely BD's business, just as their 25% bonus.
On the other hand, you sparked another question - does MS pay a different rate to each airline or does it pay a set amount to BD, UA, LH, etc and they instead foot the mileage in the appropriate denomination? This is probably more of a Star Alliance question, but would be interesting to know.