I don't know about US employment law but in UK employment law long hair is not grounds for turning down a job applicant.
That's not to say employers won't think less of you or judge you if you have long hair but if they ask you to cut it, you say no and they terminate an interview that is cause for a court case. One of my friends who was, at the time, working in catering had an interview that went like this:
Employer: "Your CV is adequate for the job but we were wondering, would you be willing to cut your hair?"
Friend: "No, sorry I like my hair as it is now but I am willing to tie it back for health and safety."
Employer: "Sorry, I just don't think you're the sort of applicant we would be looking for. Best of luck."
He ended up taking the employer to court and winning the case on the basis that he was willing to compromise with a hair-net/tied back hair. It just depends on whether the interviewer is shrewd enough to cover their tracks and not show themselves up as acting in a discriminatory manner when they're hiring.