Was smoking banned because of passenger safety or occupational safety?
If you look at the part of the Federal code (49 U.S. Code § 41706) that actually instantiates this :


  (a) Smoking Prohibition in Interstate and Intrastate Air
  Transportation.— An individual may not smoke— (1) in an aircraft in
  scheduled passenger interstate or intrastate air transportation; or
  (2) in an aircraft in nonscheduled passenger interstate or intrastate
  air transportation, if a flight attendant is a required crewmember on
  the aircraft (as determined by the Administrator of the Federal
  Aviation Administration). (b) Smoking Prohibition in Foreign Air
  Transportation.— The Secretary of Transportation shall require all air
  carriers and foreign air carriers to prohibit smoking— (1) in an
  aircraft in scheduled passenger foreign air transportation; and (2) in
  an aircraft in nonscheduled passenger foreign air transportation, if a
  flight attendant is a required crewmember on the aircraft (as
  determined by the Administrator or a foreign government).


It is the presence of a flight attendant that triggers the non-smoking requirement. Thus, one can safely assume that it is an occupational safety concern (sidestream smoke) that forced the regulation change.