This isn't QA over food. It's not like the butter was contaminated or something.

It's QA over packaging.

And you have no evidence that this was motivated by trying to "save money by doing less QA".

US food manufacturers generally do their absolute best on QA because recalls are super expensive and the headlines are bad. But companies are made up of humans who are never going to be 100.0000000% perfect.

shouldn't the question be, how the f does a manufacturer mess up the packaging of BUTTER. something they have presumably been selling for decades?

The government could apply a fine, and shops selling that butter could put up a sign warning that butter is made from milk.

That achieves the same result without the destruction of perfectly good food.

Someone buys it, then resells it without the putting up the warning sign. Who is liable: the manufacturer or the person reselling it? (I've seen plenty of small convenience stores reselling food simply because their suppliers don't offer the same prices to them as they do to large grocery chains.)