From the FAQ:

Why can’t U.S. citizens, residents or entities purchase EOS Tokens?
It was decided that U.S. citizens, residents and entities should be excluded from purchasing EOS Tokens in the token distribution because of some of the logistical challenges associated with differing regulations in the many states of the United States of America. block.one does not believe that the distribution of EOS Tokens or the EOS Tokens themselves are securities, commodities, swaps on either securities or commodities, or similar financial instruments. The EOS Tokens are not designed for investment or speculative purposes and should not be considered as a type of investment. Nevertheless, U.S. citizens, residents and entities should not purchase or attempt to purchase EOS Tokens.