How did it become "normal" for sellers to pay the buyer's agent?

[Jesse Smith]

I'm in the process of selling my primary residence FSBO. We are in a very popular neighborhood, so for now, we won't be going through a listing agent. We are also real estate investors, and we've been on the buy and sell side of many transactions. I won't need help navigating the offer/contract/closing/etc process. However, I get constant calls from agents asking to list, or asking for commission if they bring a buyer.  

I understand the value added by a listing agent - the have access to the MLS and marketing tools that I don't have. I also understand why a buyer's agent is valuable to to the buyer.  They help the buyer find a suitable house and walk them through the process of the transaction.  

So, my question is - Why are sellers expected to pay for a service for the buyer, especially when the buyer's agent's fiduciary duty is to the buyer?  If a buyer wants someone else to do the legwork for them, shouldn't they be the one to pay for that service?