Some service items are purely on a time basis and brake fluid is one of them due to the fluid absorbing moisture which 'could' affect system performance.
(I have to say in over 40 years driving and maintaining some of my own cars this has never been an issue afaik - but hey, BMW know better don't they?
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, they could have least suggested delaying it until the next proper service in 3/4 months/3000 miles or doing the full service early, they did neither of course, too smug!
Oh, and have you found the brake fluid reservoir yet?