Coinbase has opened 20 Million accounts for this reason. It is regulated as a 'money transmitter' which is not an exchange – but is based in San Francisco so at least provides Americans the illusion of credibility. Being registered and operating in the US is not only required by US regulations, it's a comfort factor for US clients. Frauds from overseas which target Americans are hard to prosecute and difficult to collect if any money is owed. Some rules even prohibit US Citizens from opening Foreign brokerage accounts. In any case, eToro's move is a strong signal that the US market is ripe. While the US is a big market, it is also an expensive place to operate, especially for foreign firms who may not speak English as their native language, and who may not understand the complex US rules.
The emergence of Crypto currencies is interesting on a number of levels. It is not clear yet how the regulatory regime that makes rules will take shape. The SEC and CFTC dispute over which parts of Crypto Currency will remain in their purview. In Malta, they have passed regulations specific to Crypto Currencies naming DAO organizations "Technology Agreements" which even go as far as allowing robotic decision making in regard to transfer of real estate.