If I remember correctly, the pure food and drug act was worded very broadly in order to ban everything (i.e. copious amounts of rat shit/rats) that could possibly contaminate your food in one go. Unfortunately, the little prizes are technically non-food stuff inside of food, which was banned somewhere in the act. Fortunately Polish/German/Russian/Etc. speciality stores could give less of a fuck and still import them anyway.

MFW Americans don't have Kinder eggs

I pity you, it's fucking delicious.