There are cases specifically intended not to follow that rule. For example, when my advisor takes on undergraduates for research projects and they haven't done research before, he does coax and prod them, and he encourages the more experienced student members of the group to do the same. In that case, the result doesn't matter so much; the project, at least initially, is meant to be an experience in which members can develop competency and motivation. Some amount of hand-holding will be necessary.

Often, teachers think they're creating this same sort of environment by assigning group projects. However, the project is structured so that the grade is still the most important thing. For a student assigned such a project, it's important for em to realize that ey has no moral obligation to go along with the teacher's implied intention for the project. Developing that sort of mindset is then helpful down the road, when one is involved in other projects where the result is most important.