It's like making synthetic foods, but by first taking the components out of natural foods so you can call them natural.
It's the easiest way to get 0%, 0.5%, 1%, 1.5%, 2% etc milk. I don't think you could do it without starting all of them from 0% milk. Especially considering you have to compensate the lack of taste from fat with extra lactose. And you can't use "added sugars" without raising major red flags with consumers.
So you use lactose. Because it's already in milk. And you don't have to say you've added it.
Source: my friend who studied food science.
"I don't think you could do it without starting all of them from 0% milk" is not a valid basis for facts. I don't want to come across like a meanie, but no one cares whatever facts you make up in your mind using (pseudo) common sense. Milk production is very well documented and regulated.
I doubt it is cheaper than a measure and blend process would be, so I doubt the majority of milk is so heavily processed.
It could certainly be that some milk is being separated down to constituents and ingredients added back to other milk, but that would be to top off a blend, not the default process (because separation obviously costs more than doing nothing).
But orange juice isn't healthy before or after processing, so the point is lost.
http://www.scientificamerican.com/article/what-is-the-differ...
I encourage you to read "Swallow This: Serving Up the Food Industry's Darkest Secrets" by Joanna Blythman. Most food if not every food that comes in a cardboard box has been assembled from parts, often from various parts of the globe. Vitamins and antioxidants are added to every processed food, iron is added to breakfast cereals, flavourings are used to change your perception of the food, caramel is added to meat to give it a nice brown color, "natural" extracts are in fact extracted using solvents, of which traces can be left in the food, and there is no need to put it on any label, because solvents are processing aids.
It's really much worse than you think unfortunately.