As I understand it, the hikers were warned against hiking the trail due to the fact that park rangers knew that a sow with cubs was in the area. They proceeded to hike the trail, nonetheless. Once it was determined that the sow did not feed on the man who was killed, the rangers decided to allow her to live.
Firearms are permitted in national parks. However, if one carries concealed, the appropriate permit(s) with reciprocity are required. Furthermore, discharging a weapon in a national park is illegal.
More info here:
http://www.nps.gov/fobu/parkmgmt/upload/imr_gunsparks_brochure_2-2010.pdf