The legality of electric skateboards is regulated at the state or city level. The federal government does not have any laws prohibiting the use of electric skateboards country-wide.
Legal disclaimer: I am not a lawyer, and this article is not to be construed as legal advice. Use common sense, and always check your local laws and ordinances to ensure you are in compliance. Neither E-Skate Central nor its authors are liable for incomplete or incorrect information.