Why is Janie considered a feminist if she is silent and waits till the end to decide to fight back?
I'm not sure to what section you're referring. If you're talking about the entire book, I have to disagree with you. Janie stands up for herself at several points throughout the novel. A better argument against her perceived feminism would be that she needs a man (Tea-Cake) in order to fully define herself as a woman. If you consider her killing Tex-Cake "Fighting back", again, I have to disagree with you. She kills Tea-Cake because he is infected with rabies, and is attempting to kill her. It doesn't have anything to do with some awakened self-realization or knowledge of her power as a woman.
However, if you're referring to her life with Joe Starks, then I would argue that yes, she remains silent. But you need to take cultural context into consideration: she is a woman, a black woman, who has no rights to speak of in this society. She imagined that her life with Joe would be free and easy, & to some extent it is. That is the financial aspect of the relationship. Joe is a successful man, and Janie reaps the material benefits of that success. But I think that as soon as Janie realizes what her life has become, & how much Joe hurts her/oppresses her, she stands up for herself. She tells him off, in front of the other men no less. After that, she doesn't let Joe tell her what to do or control her in any way. In fact, the power in the relationship shifts from him to her, and she proves herself a strong woman of the times.
It's importnat not to judge Janie according to our contemporary standards of feminism. We must understand that any attempt to lead an independent life was a strong statement for women in the 1930's.