I'm still stuck on the bit about first impressions:
The discussion started from a general, abstract elsewhere question. People raised a few obvious human-nature related issues with how a person who may have body decoration is perceived, and how that perception might be negative in FIRST IMPRESSION. As soon as your case of getting helpful, knowledgeable service happens we are no longer on first impressions, and all would agree that how the person acts and delivers what the job calls for is far more important. That supersedes a person's first impression.
When a manager hires someone, they certainly don't do it solely on first impressions, they have an interview. So the manager understands that going beyond first impressions is important. The same applies if an employee is hired with normal hair colour, proves themselves over six months, and then decides to dye their hair or get a piercing (etc.). The manager will not suddenly change their impression of how hard a worker they are.
So who is it that doesn't look past first impressions? In the case where the manager decides he/she cannot hire someone because of hair colour, perhaps he/she is thinking of customers who will not go past their first impressions. So the manager, who knows that it is important to look past first impressions, because not all people that look "professional" act professional, assumes that a large majority of his/her customers will not look beyond first impressions and hires with this in mind.
Now to the question of "fair". Is it fair for a person to be judged suitable to carry out some ability (such as selling magazines, making coffee, delivering advice) based upon a criteria unrelated to that ability? I would say no (and it doesn't sound very practical to me, either). But perhaps what defines it as "unfair" is that there is no logical relationship between the two, so to use hair colour as a factor as to how good the service will be is poor logic - it's not a way to make an accurate decision. And in the example above, the manager I described thinks it is important to look past first impressions.
What if other people judge a person in their role based upon an unrelated feature? If our manager discerned that this was "unfair", he/she would not use it in their hiring process, no matter what customers thought, because he/she would not act "unfairly" just because customers were being "unfair". Is it fair to make some extra profit because customers discriminate against hair colour, piercings, tattoos, etc.?