Apart from the asinine war drugs, risky foods, weapons, counterfeit goods, and dangerous devices, ordinary prohibited goods aren't arbitrarily decided by fiat of Big Mother.

Kinder Surprise were banned due to the letter of the 1938 FFDCA law as "non-nutritive object".

There's usually a regulatory reason, such as homologation on automobiles. There are some protectionist laws around types of automobiles that can be imported.