The US Embassy in London has given the following concise overview of E1 and E2 type visas.
The Treaty Trader and Treaty Investor visas were established to facilitate and enhance economic interaction between the United States and other treaty countries. They were not intended to serve as a means for foreigners to retire or merely reside in the United States. U.S. law (see paragraph 101(a)(15)(E) of the Immigration and Nationality Act) explicitly states that E-1 visa holders must enter “solely to carry on substantial trade” and E-2 holders “solely to develop and direct the operations of an enterprise” in which he or she has invested. Moreover, these visas are nonimmigrant visas and thus temporary. Treaty trader/investor visas can be renewed or extended only if the investment or trade continues to meet all applicable requirements of U.S. immigration laws and regulations. Persons wishing to remain indefinitely in the United States should apply for the appropriate immigrant visa." -US Embassy London