Buyer can decide whom they want to work with and change their minds. I've had Buyers working with me decide to work with another agent and vice versa. Fwiw, I always ask the latter to please inform the other agent that they've decided to work with someone else...I want my Buyers to be ethical as well.
But it is unethical for an agent to induce a Buyer to use them to write an offer instead of using the agent they've been working with. That agent should be reported to the Department of Real Estate and to whichever Board of Realtors they belong to.
I think that if I were a listing agent and a Buyer who had seen the property wanted to write the offer through me, there would have to be some pretty extenuating circumstances to make it happen, along the lines of gross incompetence or lack of ethics of the previous agent, and even then the first call would be to my own manager, to be followed by communication with the other agent's manager. In all likelihood, I would wind up cutting a referral fee to the other agent...and that's a worst case hypothetical scenario.
Yes, this would most certainly be considered unethical. Whenever I am speaking to anyone who may be a prospective client, asks me to help them view a propert, etc., I ALWAYS ask them if they are working with another agent, and if so, I tell them that they really should be going through their agent in their property search..
Coldwell Banker Residential Brokerage
Sherman Oaks, CA