I had a boss who said he only hires a certain ethnic group because they always take on more work and never say no.
He had 2 of these people end up in hospital and a formal accusation of bullying against him... my conclusion was, he'd narrowed on a culture that is averse to social challenge / conflict. Where saying "no" would be socially unacceptable.
But he wasn't aware of the flip-side... that the manager is then responsible for judging the situation and not making direct requests that are impossible to refuse.