Virtually every car has to have a bi-annual brake fluid change, nothing to do with BMW, it's down to the fluid spec as dictated by the fluid manufacturers.
Same happened to me too, I took the car in for service, you know - planned my life around not having the car, etc, etc, and all they did was the brake fluid! What a waste of time,
Didn't you discuss with them what they would be doing in advance? The car even tells you what's due!!