This is because the film negative tool alters the raw values in the very early stages of the processing. Strictly speaking, it applies a different exponent to each channel value ( details here ). The white balance tool operates later in the chain, immediately after demosaic, and it does not change the raw values, because it works on a different memory buffer (the demosaiced, “working” image buffer). So, the white balance setting cannot affect the input vaules that get exponentiated in the previous step.
it is unique in that normally, raw values are never altered (except for very small changes like dead pixels filter or dark frame diff). You can think of the film negative tool as a “preprocessing” that is applied to your picture before feeding it into RT; if you change the exponents, you basically have a different picture, and you must start all over again 