I feel like the difference is about the product. On search engines its like you are browsing a catalog or magazine. You know what is an ad, and there are many options to choose from. With a Chatbot it is more similar to talking to a person where they give you a direct answer to your question (instead of just handing you a piece of paper with a bunch of options). I think the format makes all the difference. You expect when you are talking with a "person" to be given un manipulated information. You don't expect this person is taking money in the background to influence what is said. You do know politicians purchase advertising too??