Redfin charges 1% for selling the house. They still pay 3% to the buyer's broker. The total fee is therefore 4%, versus the normal 6%, where both sides get 3%.
Why doesn't Redfin force you to use their buyer's agent? Buyer's agents are regarded as the ones at the bottom of the totem poll. Hardly anyone signs a contract with a buyer's agent. You almost always do with a seller's agent. It sounds like they've taken on the harder piece first...
Also, Redfin gave me 1.5% back when I bought a house. Ka-ching!
If you use a broker that returns 2% when purchasing the property, this basically drives it down to 2%. Which is far more reasonable than 6%.