It's not a rule. The article the OP was reading simply said that applicants should be 'willing' to cut their hair. Implication being, would they cut their hair *if* it was needed to acquire the job.
Lady Sunrise of the Prairies in the Order of the Long Haired Knights
Daddy's Girl Hair Journal 1b/1c-M-ii Knee Length (60")
This is strange. I have a friend who was a flight attendant. She had around waist length hair and it was never mentioned that she had to cut it. Although as others have said she did have to look 'business sleek' so she usually wore it up, in a bun or a ponytail. So I've no idea why your being asked to cut your hair or at least if your willing to.
With regards to would I cut my hair for a job etc. Being a guy I wouldn't be surprised to get this. But my hair is now part of my personality etc so I couldnt cut it. I would simply suggest that I would wear it up etc but never cut it.