Are power supplies for USB drive enclosures required to have UL certification?
Does the power supply plug into the mains?

I'm not sure if it's illegal to go without UL certification as UL aren't a federal requirement (although I believe it is a requirement to have some sort of certification, UL or otherwise) It does make the product look dodgy. It is certainly illegal to not have FCC/CE certification especially as switch mode adapters like those emit hideous amounts of EMI unless properly designed.