In a nutshell: a real state agent is an agent, which has a specific legal meaning and legal requirements, and that relationship can't just be hijacked by posting someone else's listing.
>most will show you homes for free
I'm still looking for that real estate agent that does anything "for free".
Meanwhile in the past when we were looking, a mediocre one tried to get us to sign an exclusivity deal that we'd agree to use him until we chose to cancel (no stipulations there at least, though legally I don't know if he could have, but still, into perpetuity unless we canceled) for -any house in the entire state-.
If neither a renter or the landlord have an agreement to pay an agent, why would the agent be owed any money?
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.