This is just a guess, but I don't think this is a food safety issues, its more a case of 'this food is gross now' issue.
When I was a teenager I worked in a frozen food processing plant. The freezers were giant boxes where freon was condensed on gigantic coils into a liquid and then the liquid freon would rain down on to the food passing under on a conveyer belt. The vegetables were frozen instantly and it was quite tasty - frozen just a few minutes after being cleaned.
As industrial freezing is very quick and such flash freezing, the quality of the food is preserved in a pretty fresh state. [its horrible for the ozone in the atmosphere but this was in the 80s and I hope they changed the process].
Milk is a mixture of water based nutrients and globs of fat, which give the milk is homogeneous white color and its smooth texture. Freezing milk in a typical refrigerator would take hours and ice crystals would form and separate out the two phases - the fatty portions would separate. Try it with a glass of milk and see. This is the same reason that you don't re-freeze ice cream, which has even heavier creme proportions (even fattier). The texture is completely gone.