Receiving milk for working in harmful environment
I know that in India there was a very similar law extant at least back till the 1990's when I had encountered it. Not sure if it is still on the books. 

The association with the USSR makes sense: during the cold war a whole bunch of Indian factories were constructed with Soviet aid. And the conventions, technology, etc. came along. 

I had heard of several alternative explanations for the milk-law; not sure which (if any) were true. Maybe partly all were. 


The Vitamin D or Calcium in milk competing against absorption of Lead or some other nasty element, I forget which the workers were exposed to in chemical factories.
The Milk just contributing to the general health of factory workers in an otherwise grossly malnourished nation (remember, this is cold war era India). Presumably due to the attendant health hazards of chemical factory work, keeping a worker there healthy was more crucial.
Just having the stomach full of a competing innocuous substance (e.g. milk) reduced the propensity of competitive absorption of other nasties in accidents, fugitive exposures etc.