first of all welcome to Cruise-Chat.com.
The Jones Act of 1920 doesn't affect cruises in general, it is the Passenger Services Act of 1886 (!) that affects all cruises calling at ports in the United States.
Wikipedia has a good explanation here
The Jones Act, however, affects and significantly reduces the possibility of operating cruise ships under the U.S. flag. Wikipedia has more detailed information on this as well, here
Hope this helps.