There is so much that can be stated in connection to this question. Let me give you a few points.
First, it is important to note that this book was published right after the Fugitive Slave act of 1850. This law made it illegal to help any runaway slave. From this perspective, this whole book was a direct criticism of this law.
Second, some have stated that the kindly treatment of slaves by the masters Shelby and St. Clare is a weakness in the novel, but there is another perspective. The very statement that these character do not speak out against slavery shows that they have moral failings. This is a powerful literary device.
Finally, we need to appreciate the point that this work is rooted in Christianity. This point is a powerful literary tool to show the complete incompatibility of Christian faith and slavery. This shows the hypocrisy of the people.