to discourage resale
with the side effect of a consistent mediocre product
It's more likely to ensure that the entire batch falls within the allowed degree of variation of active cannabinoids that must be declared- ie total THC and total CBD.
Having seen the growing methods, it's most likely they are seeing a variation that exceeds the allowable +/- 20%, and that milling the entire batch allows them to achieve a standardization that falls within the allowable variation from registered chemotype.
-Chimera