In Germany the milk in plastic tastes fine, although the plastic is more like a ziplock bag with some rigidity to it so it stands up straight and pours well.

I wasn't trying to say the plastic is the cause of the taste difference. I emphasized the plastic in the US because thats been my observation. However if you check the expiration dates in the US, milk in plastic tends to expire much more quickly than the milk in paper cartons. So there is more going on than it simply being different containers.