Why are Kinder Surprise eggs illegal in the United States?

Today I Found Out:

For the uninitiated, Kinder Eggs are a chocolate treat widely available throughout Europe, Mexico and Canada, with the company that makes them, Ferrero (perhaps better known in the U.S. for being the makers of Nutella), selling a whopping 1.5 billion of the eggs per year. Where they don’t sell any is the United States, where the eggs are indeed illegal (though something of a blackmarket does exist for them). So why is a beloved candy the world over explicitly banned in the land of the free?

I knew that Kinder eggs were illegal in the US (and it’s a point we Canadians often use to show our “superiority” over America) but I didn’t know the legal reasoning behind it.