Just because you personally disagree with the goal of a grassroots movement does not mean it is a fake grassroots movement.
A grassroots movement is very simply a collection of people, usually belonging to a community with a shared interest, who work together to publicly advocate for a particular cause. This is contrast to a powerful or moneyed interest that lobbies for a cause that usually only benefits a small group. When a powerful or moneyed interest is paying large groups of people (or alternatively bot farms) to manufacture the appearance that a grassroots movement is supporting their cause, THAT is astroturfing. The agreeability of the cause has nothing to do with how the strategy gets labeled.
You have such a tenuous grasp on the meanings of such basic words that you might want to consider hesitating before referring to other people as idiotic.