In the US, advisory labeling (i.e., "may contain...") isn't codified in law or regulation; it's something that the FDA basically recommends as a good practice w

In the US, advisory labeling (i.e., "may contain...") isn't codified in law or regulation; it's something that the FDA basically recommends as a good practice when cross-contamination may occur even though good manufacturing practices (GMPs) are being followed. For example, you might have a line that processes milk and dark chocolate, which you sanitize according to GMP, but which may still contain traces of milk. Crucially, the FDA expects that advisory labeling will not be used to get around practices such as sanitizing equipment, segregating allergens from other foodstuffs, and so on; they exist as a "we've done our best to make sure you won't die of anaphylactic shock, but caveat emptor nonetheless" warning. So what you've got here are companies that haven't had to comply with GMP requirements; decided that they don't want to; so instead just slapped a major allergen into their existing products with functionally no warning to consumers who believed the products to still be allergen-free. Which, sure, GMP compliance isn't cheap and it is complex, but this is a pretty cold-blooded way -- somewhere these companies surely have PowerPoints that do a mortality, legal, and financial risk analysis of their decisions -- to do an end-run around laws and regulation.