I think you see this only from the point of dynamic range and ignore other concerns of real life raw image procressing. When the raw values are increased, more raw levels are required to maintain continuous mapping. Such increase can happen already simply due to the WB application and adjustment specifically aimed at redistributing the levels increases the tendency to posterization.
Noise dithers tonal transitions. No matter how you process the image, no matter how the levels are stretched or squeezed or redistributed, the noise is transformed in exactly the same way and continues to dither the tonal transitions, provided the amplitude of the noise sufficiently exceeds the quantization step in the original data.
I invite you to exhibit for us an image where the raw data are not posterized and the processed data are. It will not happen absent a truncation of the bit depth; the mathematics dictates otherwise.