In the U.S., at least, I've never known a seller's agent to do that (having done a good number of real estate transactions). In fact, in my experience I find it to be the exact opposite. Seller's agents are incentivized to not work with buyer's agents, if the situation arises where they're approached directly by a buyer.
If the seller's agent is comfortable acting as the intermediary, and recognizing the limits of conflict of interest, then it's in their best interest to keep all of the 6% commission instead of giving half of it to the buyer's agent.
When looking for a place in New York, we were straight up ignored by most sellers until we signed a contract with a buyer's agent. Apparently, people without an agent are perceived as window shoppers / tire kickers who are not serious about buying and whose presence wastes everyone's time. A buyer's agent signals that you have serious intentions.
