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.
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.