This happens normally when you aren't staying in the city and need to get to the airport. When I stayed at IC Qiandaohu and IC Huizhou they asked for more because there aren't cabs waiting, and normally the resorts are frequented by those with cars.
In Beijing, I really think that you don't need to do this, and can just walk outside and find one. Your hotel can do this for you, and by doing this they can send one of their employees outside to hail one (what they often do....). I think the 50 rmb fee comes from them having to reserve the cab and have it waiting for you at the hotel at 9am. You can also do it yourself, and shouldn't be too worried about it - just give yourself enough time to do it