Yours is a very good question. The Broker (let's call them ABC Realtors) is the umbrella under which the Listing Agent (LA) and the Buyer's Agent (BA) work. For the privilege of working at ABC, the LA and BA pay anywhere from 10% to 50% of every dollar they earn, and ABC provides an office, a BROKER'S license, staff, advertising, marketing, supplies, management and oversight, and in many cases, in house counsel. All of those things are to benefit you, the client.
When you agreed to list your home, you made an offer of a fee to the Listing Broker, and you made an offer to be paid to the Cooperating Broker. (Usually 3% and 3% in California, but not fixed by law). Your agent listed the home under those terms, and offered it in the MLS. It was a contractual offer.
Had the Buyer's Broker and Agent not been ABC, but rather XYZ, you would happily (I presume) paid the fee you offered. All parties performed as you expected, and you got the desired result, a sale.
The fact that ABC is the Broker of Record for the LA and the BA in no way mitigates their cost, liabilities, etc. They have the same responsibilities to the Buyer and the BA as XYZ would have, and as such, deserve their fee.
From another point of view, think of it this way: you hired ABC and your LA because, to you, they were the best person for the job. How fortunate for you that you got a BA with the same level of expertise and professionalism standing behind them!
Good luck, and thanks for the question.
The Bremner Group at Coldwell Banker
Web Reference: http://www.TheBremnerGroup.com