The doctrine of karma is extremely important in Buddhism. Not only that it explains how things work in the world, why we are what we are, why we have this form (body), but it also assures us of our free will. We are what we think (karma of thinking), what we say (karma of speech) and what we do (karma of action or body) and we can always change this karma and thus create a more peaceful and pleasant way of life.
Apparently, by saying that sentient beings cannot free themselves from birth and death by their own power, it seems that Jodo Shinshu misinterprets or does not totally accept the doctrine of karma. However, Jodo Shinshu accepts fully the teaching of karma, just that it sheds light on a very important aspect that many Buddhists usually tend to forget.