I have only dealt with agents for apartment rentals in NYC, and there, the landlord hires the agent and agrees to pay them a fee, just like a house seller would agree to.
If neither a renter or the landlord have an agreement to pay an agent, why would the agent be owed any money?
In NYC, the renter usually pays the fee too, but that is simply part of the rental agreement from the landlord.
The person that agreed to pay agent is still the landlord. In times when supply of apartments exceeds demand from renters, landlords have to pay the agent from their pocket.
But the point is that in all cases, someone agreed to pay an agent. The agent did not simply materialize and obtained a right to collect money from someone.
If neither a renter or the landlord have an agreement to pay an agent, why would the agent be owed any money?