Can airlines choose whether or not to enable MCAS on their fleets, and if so, have US airlines chosen to do so?
No, MCAS was required for the 737 MAX to be certified by the FAA. It cannot be deactivated by any airline, including airlines in the U.S.



Source:


  "M.C.A.S. was necessary then for the airplane [737 MAX] to be
  certified by the F.A.A. to have met all of the regulatory design
  requirements for stability and control."
  
  https://www.nytimes.com/2019/02/03/world/asia/lion-air-plane-crash-pilots.html