It’s not spiteful, it’s profit maximizing. Being labeled as “processed in a factory that may process sesame” doesn’t remove your liability under the new law. It either has sesame as an ingredient or it doesn’t, and if it doesn’t it must not contain sesame.

What if you label it as definitely containing sesame but don't actively add sesame?

You cannot add an ingredient to your list which isn’t actually there. It would violate another law.

I wouldn't say "spiteful", as the OP, but it's certainly selfish and antisocial. Those companies simply choose to maximise their profits at the expense of a minority of their clients, for whom they obviously don't give a shit.

Note that we're talking about companies big enough to have factories where they make several different kinds of food item. So those are the same people who happily sell over-processed food with excess quantities of salt, sugar and fat. Not only do they not give a shit about the minority of their customers with allergies, they don't give a shit about the majority of their customers, either. They just want to sell shit and make $$$!!!

This legislation backfired because people are assholes and put their profits above everything else, not because regulators are idiots who can't foresee the consequences of their decisions.