Easy, Khan was either genetically engineered to have no facial and body hair (because hairless bodies are considered perfect), or it was an accidental side effect.
Sikhism is also a religion that, as always, preaches love and peace. If he ignores one thing, why can't ignore other things?
Like the fairy tale that Muslims aren't allowed to drink alcohol. I know Muslims that pray 5 times a day and still give a shit about that prohibition.
Had they said he was a Catholic, imagine how many different arguments you could make about what he was supposed to do and be and what not.