what people mean when they talk about not liking a certain thing in a piece of fiction because they think it was manipulative?

On the one hand I'm genuinely confused. On the other, what were they expecting? That's what fiction IS. It's not a historical account; if I write that someone has died, it's because I want the reader to feel sad or relieved or whatever, and the tone and wording of the scene will support that. When I do that, or simply when an author puts any element into a story that isn't required for a factual statement of events, it's intended to get a reaction out of the reader. It's manipulative.

Is their problem that it isn't subtle enough? I don't get it.