I think it's something that happens during pasteurization and/or homogenization. I usually only look at store brand milk, and it all says it's free of rBST. Whether it's organic or not doesn't matter.
I know this will sound a bit wacky or dangerous, but raw milk works just fine for me - not pasteurized, and not homogenized. It's tough to get though, and even illegal in some places. There have even been police raids done on farms that sell it, even though they plainly state what they're selling.
If it's collected hygienically, I don't think there's much to worry about. Pasteurizing is of course easier for industrial farms, especially back when the process was introduced. "Make it hot, and it won't make people sick."
Or maybe try cleaning the manure off of the teats and sanitize them prior to milking. Remember, we're the species that had doctors who would break from doing an autopsy to go deliver a baby, and then wonder why so many mothers and infants died a few days later. Germ theory wasn't always common knowledge.
Anyway, regular milk results in the generation of lots of gas and various other interesting materials in very unpleasant and inconvenient quantities.
I'm
fairly sure it's not psychosomatic, otherwise I've got a brain that can spontaneously generate matter from nothing.
Lactase tablets sort of help, but they're not a 100% guarantee.
But raw milk is as benign as drinking water, at least as far as my intestines are concerned. I guess it helps with gut flora or some such thing, because for about 2 weeks afterward, store-bought milk also doesn't cause any problems.
*
shrug*
I don't buy it frequently though - there's only one place I know of in the area that sells it, and driving all the way there and back again burns through at least 2 gallons of gas. I just don't really like going to a much trouble for anything related to food. (Put it this way: If I could instead absorb usable energy from a wall outlet, I'd gladly take that route.)

It does seem to violate the spirit of the whole thing.
Not only the taste, but the texture is also mimicked.
I'm eager to see what they'll do when or if we're able to grow animal muscle tissue itself on an industrial scale.
.