Buyer paying additional commission upon sale of FSBO

[Kay March]

I'm selling one of my rental homes as a FSBO, and we close on Tuesday. I learned from the settlement statement that not only will I. the seller, pay the buyer's agent 3%, as we agreed, but the buyer will also pay their agent $3,000 (this is apparently a flat fee). Please understand -- I know that all commissions are negotiable. My question is about common practice, or the usual way of doing things. How often does this happen, and why would the buyer agree to do this?