[andy81]

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.

[II2II]

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.)