The Wisconsin dairy lobby has a well-deserved reputation for things like this being hidden in law.
The wiki page for Margarine tells some of the story https://en.wikipedia.org/wiki/Margarine
But if you do a search for Margarine Act of 1950 (yep, Truman signed such a thing) and see the many articles, you will find that margarine had quite a battle to become readily available in the U.S. And the Wisconsin dairy lobby held out the longest of any state...until 1967.
Looks like Kerrygold ran afoul of one of those obscure statutes.
They take their dairy products very seriously in Cheesehead land.
Pennsylvania’s dairy lobby is just as bad.
The retail price of milk here is set by a state regulatory board, and has been since the Depression.
You can drive to Ohio and find milk for a buck-and-a-half less per gallon.
But if they catch you they’ll confiscate your vehicle. No joke!!